Designing an AM Generator Circuit with BF494 & IFT

[dexterdev]

Hi guys,
I have a circuit for AM generation using BF494 (beta = 160) and and an IFT (455 KHz) etc. The problem is that I don't know how we arrive at the resistor and capacitor values in this circuit. Can anyone help me withe design of this circuit. Vcc = 12 V. I am attaching the circuit schematic. I tested the circuit with values given in the image attached, it is working very well. When I took the 1Mohm from circuit still the AM circuit works. I wanted explanation for the working and design of this circuit.

I am also inserting the design procedure currently I am having with the image , please check it for any errors and correct me.

TIA

-Devanand T

 

[NascentOxygen]

Yes, that 1MΩ doesn't comply with the design procedure outlined. A correctly chosen R2 would give I_C of the desired 1mA. Without R2, my calculation shows that I_C will rise to only about double this (the rise is limited by the heavy feedback via R_E) . As you discovered, the circuit will likely still function okay.

Coupling capacitor C1 is chosen so that its reactance (at the lowest audio frequency of interest) is lower than the resistance it "sees" at the emitter terminal.

BTW, if this is a homework question, it should be in the homework section.

 

[dexterdev]

Thanks for the reply.
Can you please explain the working of this circuit?

 

[davenn]

where did this circuit come from ?

Its a bit unusual to be applying the modulating signal to the emitter rather than the collector

Also the transmitted AM out should be coming off the secondary of that IFT transformer
rather than the common point between the primary and the collector

the circuit looks a little strange ... its sort of a Colpitts oscillator but not quite

Dave

 

[dexterdev]

This circuit is used in my college for AM generation laboratory . But no one says where they got it or how ? I am also having the same doubts, but it works.

 

[NascentOxygen]

Thanks for the reply.
Can you please explain the working of this circuit?

At first glance, it is not clear to me exactly how it works. Can you tell me whether this single stage oscillates by itself, or do you feed the 455 KHz IF oscillator signal into the IFT?

Regardless, I do wonder how linear it will be.

When you tested it, was that only the DC conditions you checked, or did you examine its operation as a modulator?

 

[dexterdev]

The only thing I tested was DC conditions and never fed any 455KHz carrier. This circuit itself generates the output when I feed 1Khz signal at emitter.

 

[NascentOxygen]

To cause it to oscillate, capacitor C gives positive RF feedback, this base feed coming from a low-impedance tap on the inductor of the collector's tuned circuit. It must be a tolerant oscillator, as you say it appears to continue to operate even when one of the biasing resistors is removed — though that change will cause the Q-point to move to be close to Vcc, leaving very little room left for V_CE amplitude excursions due to oscillation.

 

[skeptic2]

Have you looked at the carrier on an oscilloscope? I find it hard to believe that you can get anything close to 100% modulation without severe distortion. For good, low distortion AM I believe you should have the oscillator and the modulator as two different stages.

After simulating the circuit in LTSpice, the highest percentage of modulation I can get without seeing distortion in the waveform is about 70%.

 

[skeptic2]

See attachment

 

[dexterdev]

Today I will try to post the waveform which I get from lab.

Actually for me now help comes only from you people. I don't know where to get a good practical laboratory manual for analog communication lab. If anyone has some idea, please give the book info.

 

[dexterdev]

I am attaching the AM output waveform snapshots from CRO.
The input is 1KHz sine wave and mod index is maximum 0.7 itself.
@skeptic2

Any way can you please explain the working of your circuit? I am desperately looking for it.

 

[dexterdev]

@skeptic2
Can you please check the simulation when 1 M ohm is replaced with 100 K ohm.

 

[skeptic2]

For an oscillator to oscillate, it must get positive AC feedback. In this circuit it is accomplished by the tapped inductor. Note that the power supply is connected at the tap. The upper side of the coil has fewer turns than the lower side an so it has lower impedance. This helps to match the lower impedance of the base compared to the collector. Since the bottom side of the coil goes negative with a positive going base, the upper side of the coil must go positive, thus providing positive feedback to the base.

The operating point of the circuit is set by the voltage divider R1 & R2. At 82K and 1 meg, the bias voltage on the base is 1M /(82K + 1M) * 12 except that the base current drops this voltage substantially. The base voltage minus Vbe gives the voltage at the emitter and that voltage across the emitter resistor gives the emitter current. When the voltage at the emitter is increased with the modulating signal, the emitter current is reduced and also the oscillation voltage at the collector. If the modulating signal were increased in an attempt to achieve 100% modulation, the oscillator would stop oscillating at every peak in the modulating signal.

When the 1M resistor is changed to 100K, the voltage at the base is reduced which causes a reduction in collector current, which in turn results in reduced level of oscillation voltage.

 

[dexterdev]

Thank you...