IF amps for a superhet AM receiver

[perplexabot]

Hey all. So I am now trying to work on IF amps for a superhet AM receiver (medium wave) and found the following figure in the ARRL handbook:

I have a couple of questions regarding these schematics.

First and most importantly, do these work for IF signals (of 455KHz with BW of 18KHz)?

Assuming the answer to the above is "Yes," these questions follow:
For medium wave can I disregard VHF and UHF parasitics and go for the simplest schematic (schematic A)?
Why is it that the collector is not connected to the center tap (but somewhere below/above)?
What is the variable capacitor's job?
Why are the inductors (of the output transformer) not the same size? Does this just mean that the transformer has a ratio of x:1?
Why is there a capacitor connected to Vcc? Is it to prevent the signal from going to the source, Vcc?

I am sorry for the long list of questions but I have tried to search for IF amps and this is the simplest of the circuits that I could find, yet I am not able to understand some of it. Thank you for reading.

EDIT: Thank you for the title rename...

 

[vk6kro]

No these won't work as IF amplifiers in a receiver. They don't have any bias resistors, so they would be very insensitive as IF amplifiers.
Not difficult to arrange, but, as shown, they wouldn't work properly.

The tapping positions and transformer ratios depend on the impedances that are connected to the transformer.

The variable capacitor is used to tune a winding of the transformer to resonance. It is intended to pass a narrow range of frequencies and reject other frequencies.

The capacitor at Vcc is a bypass capacitor. This does more than stop signal going along the power supply line.
It is a return path to ground for the output of the amplifier and essential for the amplifier to work properly.
It is just as important for an amplifier as a complete circuit is for a flashlight.

 

[perplexabot]

No these won't work as IF amplifiers in a receiver. They don't have any bias resistors, so they would be very insensitive as IF amplifiers.
Not difficult to arrange, but, as shown, they wouldn't work properly.

That is disappointing, I was hoping my IF AMP search was close to an end. If I add bias resistors will it fix the problem? I am not sure what you mean by bias resistors? Do you mean biasing the common emitter with DC to setup the quiescent point?

The tapping positions and transformer ratios depend on the impedances that are connected to the transformer.

I see two weird things about this transformer, first the primary and secondary windings are not equal. Second, the tap isn't in the center. Why play with both these parameters to adjust impedance, instead of just altering one?

The variable capacitor is used to tune a winding of the transformer to resonance. It is intended to pass a narrow range of frequencies and reject other frequencies.

I just recently learned about tuned transformers. I thought the capacitor would have to be in parallel with the transformer winding?

The capacitor at Vcc is a bypass capacitor. This does more than stop signal going along the power supply line.
It is a return path to ground for the output of the amplifier and essential for the amplifier to work properly.
It is just as important for an amplifier as a complete circuit is for a flashlight.

I see, that makes sense.

Thank you for your reply.

 

[vk6kro]

Yes, just add resistors to apply a suitable base current to the transistors.

The primary of the transformer is a tuned circuit. If you connect the fairly low impedance of a transistor collector across this, it will affect the resonance and cause reduced output and increased bandwidth.
Tapping low on the coil reduces this effect.

The variable capacitor is in parallel with the primary of the transformer. Just regard the bottom end of the coil as grounded because of the bypass capacitor.

 

[perplexabot]

Yes, just add resistors to apply a suitable base current to the transistors.

The primary of the transformer is a tuned circuit. If you connect the fairly low impedance of a transistor collector across this, it will affect the resonance and cause reduced output and increased bandwidth.
Tapping low on the coil reduces this effect.

The variable capacitor is in parallel with the primary of the transformer. Just regard the bottom end of the coil as grounded because of the bypass capacitor.

Thank you so much! Now that I know with the correct biasing these circuits work as IF amps, do you think circuit (A) will suffice for medium wave AM?

 

[vk6kro]

IF amplifiers also have AGC or automatic gain control and these amplifiers have no provision for this.

This is possible with bipolar junction transistors, but you might like to search for actual circuits as the way AGC is generated matters for it to work properly.

Most circuits reduce the bias current for stronger signals. Unfortunately, this also makes the amplifier more liable to overload. So, it is a delicate balance.

 

[perplexabot]

IF amplifiers also have AGC or automatic gain control and these amplifiers have no provision for this.

This is possible with bipolar junction transistors, but you might like to search for actual circuits as the way AGC is generated matters for it to work properly.

Most circuits reduce the bias current for stronger signals. Unfortunately, this also makes the amplifier more liable to overload. So, it is a delicate balance.

I was afraid of having to look into AGC : ( I have seen it around in IF AMP literature but have constantly swept it under the rug. That's what I get :rofl: . I guess it is time. I will read up on AGC and find a circuit that incorporates it. Thank you.

 

[sophiecentaur]

AGC (historical note):
I believe that the invention of AGC was originally due to Armand Dennis (famous and influential wildlife photographer and TV broadcaster from way back). He funded his filming expeditions on money he made from the invention. One of the first uses of feedback in electronics; where would we be without it?

 

[vk6kro]

I love feedback loops until something goes faulty in the loop. Then you get screwy behaviour which is very difficult to locate.

I have built receivers using mainly dual gate Mosfets.
In the IF strips, I use transistor radio IF transformers backwards. That is, with the input fed to the low impedance winding normally used as an output.
Surprising gain is available like this because the secondary is largely unloaded by the FET gate.

AGC is achieved by reducing the voltage on Gate 2 or the drain.

 

[perplexabot]

AGC (historical note):
I believe that the invention of AGC was originally due to Armand Dennis (famous and influential wildlife photographer and TV broadcaster from way back). He funded his filming expeditions on money he made from the invention. One of the first uses of feedback in electronics; where would we be without it?

Interesting. Thanks for the info.

I love feedback loops until something goes faulty in the loop. Then you get screwy behaviour which is very difficult to locate.

I have built receivers using mainly dual gate Mosfets.
In the IF strips, I use transistor radio IF transformers backwards. That is, with the input fed to the low impedance winding normally used as an output.
Surprising gain is available like this because the secondary is largely unloaded by the FET gate.

AGC is achieved by reducing the voltage on Gate 2 or the drain.

Yes, I was looking into dual gate mosfets! So you flip the transformer around? I see, I will keep that in mind. I will post a circuit today with AGC (maybe even soon). THANK YOU!

 

[perplexabot]

So here is my sad attempt at this AGC circuit along with an edited schematic (A), what do you think? I don't need something that is superb, I want something that will work. After I finish this project at the basic level, I will then start to upgrade it.

 

[perplexabot]

EDIT: I forgot to include a capacitor in parallel with R4!

 

[Bobbywhy]

AGC (historical note):
I believe that the invention of AGC was originally due to Armand Dennis (famous and influential wildlife photographer and TV broadcaster from way back). He funded his filming expeditions on money he made from the invention. One of the first uses of feedback in electronics; where would we be without it?

O sophiecentaur! I found another historical note about the origin(s) of AGC:

AM radio receivers
In 1925, Harold Alden Wheeler invented automatic volume control (AVC) and obtained a patent. Karl Küpfmüller published an analysis of AGC systems in 1928.[1] By the early 1930s most new commercial broadcast receivers included automatic volume control.[2]
http://en.wikipedia.org/wiki/Automatic_gain_control

Cheers, Bobbywhy

 

[vk6kro]

Mostly OK.

Signal is normally connected to gate 1 of a dual gate mosfet.

At this gate, you need a parallel tuned circuit to ground. This can use the larger of two windings on a transformer as its inductor. The IF transformers from transistor radios are ideal for this, used backwards as mentioned earlier.

This produces a large voltage step-up which gives you good gain.

You don't need R4.

I would try reducing the voltage on gate 2. One way would be to put a resistor (4.7k ?) from G2 to Vcc. Then put a NPN transistor from G2 to ground. Put a bypass capacitor, 0.1uF, from G2 to ground.
Turn the transistor on via a large resistor (100k ?) from the positive AGC to the base of the transistor, reducing the voltage on G2.

I also used AGC to reduce the drain voltage. FETs give really poor gain at low drain voltages but no extra distortion, so this is useful for gain control.

 

[perplexabot]

THANK YOU SO MUCH! I will start working on it as soon as possible.

 

[skeptic2]

Is this project primarily educational? If not, there are a number of IF integrated circuits that will do the job with a lot less effort.

 

[perplexabot]

Is this project primarily educational? If not, there are a number of IF integrated circuits that will do the job with a lot less effort.

I am working on this project in my spare time. It is "educational," in the sense that I am being educated by myself, and of course, the wonderful community here at PF.

 

[vk6kro]

Sometimes projects like this are worth doing just because they are fun to do.

Using a large IC may get you a result, but you won't learn much. It will either work or it won't and if it doesn't you won't be able to do much about it because the IC's operation is mostly hidden from you.

They still need filters and minor components, so you end up with all this stuff crowded around one IC.

At least using discrete components, you can spread it all out, see what is happening and fix it if something isn't working.

It may not be economical though.
I have an MP3 player that has an FM radio in it. Cost $7 delivered.
If I had 300 songs, it could hold them and play them back at beautiful quality, and all on a single AA battery.
How could you compete with that?

 

[perplexabot]

I would try reducing the voltage on gate 2. One way would be to put a resistor (4.7k ?) from G2 to Vcc. Then put a NPN transistor from G2 to ground. Put a bypass capacitor, 0.1uF, from G2 to ground.
Turn the transistor on via a large resistor (100k ?) from the positive AGC to the base of the transistor, reducing the voltage on G2.

So, I just finished editing the schematic as you have said in one of your previous posts. Looking at it now, I realize that the extra transistor you have told me to insert at G2 is in the common emitter configuration... Will that not in effect increase the voltage at G2, instead of decreasing it?
I also used an nmos instead of a npn... is that ok?

Here is an updated schematic:

 

[vk6kro]

No.

If there is zero base current in the transistor, the will be no collector current.

As you increase the base current, there will be a voltage drop across the 4.7k resistor, so there will be less voltage on G2 and less gain from the mosfet.

Try to use an NPN. They will start to turn on as the base voltage gets higher than 0.6 volt.

You can use any N-channel dual gate mosfet.
I have used several types like MPF121, MPF131 and BF981 but you might have to check what is available on EBay.

Check the voltage recommended for G2 for your mosfet. You can have a voltage divider and put an NPN transistor across the bottom resistor of the divider to get AGC control.

It is also usual to put an source resistor in circuit. Maybe 150 ohms with 0.1 uF across it. These go between the source and ground.

 

[perplexabot]

No.

If there is zero base current in the transistor, the will be no collector current.

As you increase the base current, there will be a voltage drop across the 4.7k resistor, so there will be less voltage on G2 and less gain from the mosfet.

Try to use an NPN. They will start to turn on as the base voltage gets higher than 0.6 volt.

You can use any N-channel dual gate mosfet.
I have used several types like MPF121, MPF131 and BF981 but you might have to check what is available on EBay.

Check the voltage recommended for G2 for your mosfet. You can have a voltage divider and put an NPN transistor across the bottom resistor of the divider to get AGC control.

It is also usual to put an source resistor in circuit. Maybe 150 ohms with 0.1 uF across it. These go between the source and ground.

Interesting, I don't think I have seen a transistor used for this purpose. Pretty cool. So this added npn is a inverter, right? When AGC is low, you will have Vcc at G2, as AGC increases G2 will decrease. I think I get it. But this assumes AGC's max voltage is Vcc, right?

Thank you for all the information.

 

[vk6kro]

Hopefully you would never have AGC near Vcc levels. There has to be a high value resistor in series with the base of the transistor, so that high levels of AGC could not do any harm.

You would have AGC applied to at least two amplifiers, so the reduction in gain would be cumulative. This should limit to AGC level.

If the receiver had an RF amplifier in it, the circuit would be the same and you would apply AGC in the same way.

AGC is mainly used so that signals of different strength produce fairly similar levels of audio output.
You can omit it initially to get quick results and add it for convenience later, if you like.

I took a 14 MHz SSB receiver, made using these amplifiers, to Nepal once. Worked well.

 

[perplexabot]

Hopefully you would never have AGC near Vcc levels. There has to be a high value resistor in series with the base of the transistor, so that high levels of AGC could not do any harm.

You would have AGC applied to at least two amplifiers, so the reduction in gain would be cumulative. This should limit to AGC level.

If the receiver had an RF amplifier in it, the circuit would be the same and you would apply AGC in the same way.

AGC is mainly used so that signals of different strength produce fairly similar levels of audio output.
You can omit it initially to get quick results and add it for convenience later, if you like.

Yes, I did design an RF amp. I have got the bare basics of AGC thanks to you. I think it is better to start without it and add it once I know the am receiver is working. Should I still include the new transformer we added to the IF amp or no?

I took a 14 MHz SSB receiver, made using these amplifiers, to Nepal once. Worked well.

That's cool. Hopefully if I get this receiver working I can move on to SSB, or maybe something else. baby steps.

 

[vk6kro]

You can just use the same circuit for each stage. Connect stages by having a low impedance transformer winding coupled to a tuned secondary.

The first transformer has the antenna connected to its low impedance winding.

Mixers are made by feeding an AC waveform from an oscillator to gate 2 of a dual gate mosfet.

I avoid feeding AGC to G2 of a mixer to avoid making the oscillator shift in frequency with changing AGC voltage.

Go slowly and get each stage working before you go on to the next one.

 

[perplexabot]

You can just use the same circuit for each stage. Connect stages by having a low impedance transformer winding coupled to a tuned secondary.

The first transformer has the antenna connected to its low impedance winding.

Mixers are made by feeding an AC waveform from an oscillator to gate 2 of a dual gate mosfet.

I avoid feeding AGC to G2 of a mixer to avoid making the oscillator shift in frequency with changing AGC voltage.

Go slowly and get each stage working before you go on to the next one.

Wow, this is so helpful. Thank you so very much :) You can make a mixer using a dual gate mosfet? That is crazy! I was going to use a ring diode topology but this will simply a lot. Thank you. I was worried about how I will connect the stages but you have simplified it with these transformers, thank you.

would this stage also work for the audio stage?

THANK YOU!

 

[vk6kro]

Dual gate FETs work well as mixers.
Don't put a bypass capacitor on G2, of course.

You don't need suppression resistors in the output of these FETs.

The FET itself has low voltage gain (ie output voltage/ input voltage) and the gain mostly comes from the voltage step up of the transformer.

There are many options for audio amplifiers. You can make your own with transistors or use ICs.
Or you can get a ready made circuit board which already works.

I have used the LM380 IC which is still available on EBay.

 

[perplexabot]

Dual gate FETs work well as mixers.
Don't put a bypass capacitor on G2, of course.

You don't need suppression resistors in the output of these FETs.

The FET itself has low voltage gain (ie output voltage/ input voltage) and the gain mostly comes from the voltage step up of the transformer.

There are many options for audio amplifiers. You can make your own with transistors or use ICs.
Or you can get a ready made circuit board which already works.

I have used the LM380 IC which is still available on EBay.

So I was looking into how to make audio amps and realized that a Class AB amp would probably be what I need. I then thought, I can just buy an op-amp and that would work. My question is, these audio amps (like the LM380), are they like op-amps but specialized in audio frequencies?

I was looking into the following components:
transformers: http://www.digikey.com/product-detail/en/2-4-6WEL/2-4-6WEL-ND/4696994
dg mos: http://www.digikey.com/product-detail/en/BF998,215/568-1973-2-ND/807487
narrow BPF: http://www.digikey.com/product-detail/en/CFWLA455KBFA-B0/CFWLA455KBFA-B0-ND/4419717
audio amp: http://www.digikey.com/product-detail/en/LM380N-8%2FNOPB/LM380N-8%2FNOPB-ND/6274

what do you think?

I don't know how to choose and use the transformers when the tap isn't in the middle? How does one use it? Is the tap adjustable or does one simply just place a contact somewhere along the coils, and that would be the tap?

Also just so I am sure between every stage and then next there will only be one transformer, right?

Finally, what should my varac range be?

Sorry for the constant questions, but I want to make sure I get this. Thank you.

PS: Just to be clear I am working on a superhet am (with carrier).

 

[davenn]

So I was looking into how to make audio amps and realized that a Class AB amp would probably be what I need. I then thought, I can just buy an op-amp and that would work. My question is, these audio amps (like the LM380), are they like op-amps but specialized in audio frequencies?

That's an approximation. They are capable of much more power out than any op-amp as well. A slightly more modern and probably more easily available audio amp chip is the LM386.
The LM380 is a very old chip

I was looking into the following components:
transformers: http://www.digikey.com/product-detail/en/2-4-6WEL/2-4-6WEL-ND/4696994
dg mos: http://www.digikey.com/product-detail/en/BF998,215/568-1973-2-ND/807487
narrow BPF: http://www.digikey.com/product-detail/en/CFWLA455KBFA-B0/CFWLA455KBFA-B0-ND/4419717
audio amp: http://www.digikey.com/product-detail/en/LM380N-8%2FNOPB/LM380N-8%2FNOPB-ND/6274

what do you think?

Those transformers in the first link, I'm not familiar with. Not sure if that will do what you want
The Ceramic filter may be OK, but it will have to be reasonably wideband for Broadcast AM something ~ 15kHz at least

I don't know how to choose and use the transformers when the tap isn't in the middle? How does one use it? Is the tap adjustable or does one simply just place a contact somewhere along the coils, and that would be the tap?

That's why I'm thinking those ones in your first link may not be useable

IF transformers are commonly tunable

Also just so I am sure between every stage and then next there will only be one transformer, right?

Finally, what should my varac range be?

Yes
What variac ??

Sorry for the constant questions, but I want to make sure I get this. Thank you.

PS: Just to be clear I am working on a superhet am (with carrier).

you probably should be using normal IF cans for AM radio Will find a link ( hopefully)

VK6KRO will probably chime in with other recommendations

Dave

 

[davenn]

OK
after searching dozens of pages
455kHz IF transformers are pretty scarce and a lot of guys try and build their own with varied success

In this link there is a box of IFT's for various frequencies
The first section shows ones for 455 kHz and their Mouser Electronics catalog part number

It would be a good suggestion to see if these ones are still available :)

cheers
Dave

 

[perplexabot]

OK
after searching dozens of pages
455kHz IF transformers are pretty scarce and a lot of guys try and build their own with varied success

In this link there is a box of IFT's for various frequencies
The first section shows ones for 455 kHz and their Mouser Electronics catalog part number

It would be a good suggestion to see if these ones are still available :)

cheers
Dave

Thank you so much for all the wonderful info you have shared. So you were saying 455kHz variable transformers are hard to come by? I certainly don't want to end up building them : ( Thank you for the link, I will be searching for Mouser transformers. I will try to research some more and you keep you updated. I am trying to purchase most of my components from digikey.com as you may have noticed, but if I can't I will try other vendors.

I meant variable capacitor for varac, sorry! It is connected in parallel with one of the windings in the output transformer in the schematics. I don't know what a proper range for it would be.

 

[vk6kro]

If you can find them, try to get the 14 pin LM380. This uses the centre 6 pins as a connection to a heat sink.

The output of these is much greater than most opamps. They also give very undistorted output.

A trick to make them stable is to connect an 82pF capacitor between the two inputs.

Also don't omit the series R/C network across the speaker. (12 ohms / 0.1uF)

You need the type of IF transformer used in transistor radios. The ones used in vacuum tube radios are not suitable.

Most of the transformers will have tappings on the windings, but you don't need them in this circuit.

You only need one transformer between stages. They should cost less than a dollar each on EBay.

 

[cabrera]

Hi, first of all I have been following this topic as I always wanted to built a transceiver. I don't know much about this type of transformer but I guess that you have to check the impedance ratio between the primary and secondary windings. The data sheet only shows a single impedance ratio (between primary and secondary) therefore I guess all are centred tapped.

 

[vk6kro]

The IF transformers used in older transistor radios have a capacitor across the larger winding and also have a ferrite slug which moves into both windings, changing their inductance.

Used with the input on the lower inductance winding, the secondary is almost an unloaded tuned circuit.

Viewed from the primary, the impedance rises at resonance and there is a large step-up of voltage from primary to secondary at resonance.
This is ideal for FET RF amplifiers which have a low impedance output and a high impedance input.

There is usually a tapping on the larger winding, but this is not needed in this circuit.

So, you can see that the action of the transformer is not just impedance transformation.