Create a two-stage amplifier without feedback

In summary, the goal is to create an amplifier that meets the requirements while keeping the power draw as low as possible. The first step is to select a working point, and then select the component values needed to achieve this point. The gain factor is then calculated, and any adjustments to the component values are made.The amplifier is then tested to ensure its operation.
  • #1
12
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< Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown >

Hi,

I have a task to create a two-stage amplifier without feedback.

It shall be made with these requirements;

It shall be connected to a source of internal resistance equal 2.2kΩ and a load of 1kΩ.

Create amplifier so that these requirements are met:

• Voltage Gain at medium frequencies = 100 ± 10%
• input resistance ≥ 20kΩ
• Lower limit frequency ≤ 50Hz
• Quite linear
• Working point is stable
• Power draw should be ≤ 10mAThe input signal amplitude is less than or equal to 20mV.
The supply voltage is 15V.
Bilde.png


As showen in the picture the type of transistor is: BC546B, so its possible to find the beta value (Typ 290).

I want to find the value of the ressistors and capacitors, to meet thise requirements above.

What is the first thing i should find out, with these requirements?

How should I start?
 
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  • #2
bergen89 said:
What is the first thing i should find out
Transistor biasing. That's a topic you should have studied before being given an assignment such as this.
 
  • #3
NascentOxygen said:
Transistor biasing. That's a topic you should have studied before being given an assignment such as this.
Thanks for your answer! :)
Would the beste think to do;
1. transistor ac analysis. (Because we know that the gain should be 100)
2. Give the ressistor values, so we have the gain of 100.
 
  • #4
Do you know the purpose of resistor RE18?
 
  • #5
NascentOxygen said:
Do you know the purpose of resistor RE18?
No, to now I try to use the t-modell (Because of the two reisstor at Emittor).
 
  • #6
NascentOxygen said:
Do you know the purpose of resistor RE18?
When the capacitor is low impedance, then RE18 parallels RE1 to create a lower emitter resistance for AC than for DC.

These resistors provide negative feedback.
 
  • #7
Take a look at this paper: http://www.linearaudio.nl/linearaudio.nl/images/pdf/otala%20low%20tim%20amp.pdf [Broken]
It is a detailed description of a very famous power amplifier and the included preamplifier.
 
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  • #8
bergen89 said:
I have a task to create a two-stage amplifier without feedback.
I do not think you mean that. Without some sort of negative feedback, you would just create an oscillator. Even the circuit you proposed has feedback - the resistor in the emitter creates what we call "current feedback".

There are hundreds of circuits on the web - most of them either outdated or specialized. I sort of liked this one, even if it uses three transistors. It is quite possible to modify it to your requirements.
http://myweb.tiscali.co.uk/norcimradiocontrol/Radio15_files/image014.gif [Broken]​
 
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  • #9
Svein said:
I do not think you mean that. Without some sort of negative feedback, you would just create an oscillator. Even the circuit you proposed has feedback - the resistor in the emitter creates what we call "current feedback".

There are hundreds of circuits on the web - most of them either outdated or specialized. I sort of liked this one, even if it uses three transistors. It is quite possible to modify it to your requirements.
http://myweb.tiscali.co.uk/norcimradiocontrol/Radio15_files/image014.gif [Broken]​
Thanks for you respone Svein. I am quite sure a wrote the wrong thing about negative feedback. I am quite new to this.

But if I am going to use the circuit above. What would be the first step to to. What I have read to now is that I should do a dc analysis, to find out if the transistor are in active mode. But should I just choose some ressistor values then? And after that do the AC analysis to get the wantet gain of 100.
 
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  • #10
bergen89 said:
But if I am going to use the circuit above. What would be the first step to to. What I have read to now is that I should do a dc analysis, to find out if the transistor are in active mode. But should I just choose some ressistor values then? And after that do the AC analysis to get the wantet gain of 100.
Some of the changes are easy, the input impedance for example. Others - the figure says that open loop gain is about 100. Can you identify the output-to-input feedback?
 
  • #11
bergen89 said:
Im quite sure a wrote the wrong thing about negative feedback.
What do the assignment specifications say about "feedback"? Can you provide a jpeg of the complete specifications? Is this circuit going to be constructed and its operation verified, or is it just a paper exercise?
 
  • #12
NascentOxygen said:
What do the assignment specifications say about "feedback"? Can you provide a jpeg of the complete specifications? Is this circuit going to be constructed and its operation verified, or is it just a paper exercise?
To goal with the taks is to:

1. Selection of working point. This choice depends on the signal strength of the output signal.
2. Selecting the component values so that the working point is very little dependent on the transistor data.
3. Calculate the gain factor, first an estimate so that one can get started and then a more accurate calculation.
4. Consider how the amplification factor can be made less dependent on the transistor parameters.
5.Calculate the maximum signal strength.
6. Calculate the lower limit frequency.

-- And one of the main thing is to do simplifications to do the task easier.

Then this requirements are given:

It shall be connected to a source of internal resistance equal 2.2kΩ and a load of 1kΩ.

Create amplifier so that these requirements are met:

• Voltage Gain at medium frequencies = 100 ± 10%
• input resistance ≥ 20kΩ
• Lower limit frequency ≤ 50Hz
• Quite linear
• Working point is stable
• Power draw should be ≤ 10mA


The input signal amplitude is less than or equal to 20mV.
The supply voltage is 15V.

Then after i have calculated the components, I am going to test it in Orcad to see if the requirements are fulfilled.
 
  • #13
Im stuck on were to begin. I have tried to do the AC analysis (T-Modell on both, because ressistor at emittor), I know the gain (100), so I hope then to find the ressistor components. But the answer does not look to good to now, but is that the right way to start?
 
  • #14
Start with DC settings. Aim for reasonable midpoint values. Then use a small-signal model to calculate the midband AC gains. Then maybe go back and fix more appropriate DC settings. You only need about x10 AC gain for the second stage and then tweak the first stage to make overall gain x100 so not particularly demanding for the high gain transistors you are using.
 

1. What is a two-stage amplifier without feedback?

A two-stage amplifier without feedback is a type of electronic circuit that consists of two amplification stages connected together. It does not incorporate any feedback mechanism to regulate the output signal, making it a simpler and less stable design compared to feedback amplifiers.

2. How does a two-stage amplifier without feedback work?

In a two-stage amplifier without feedback, the input signal is first amplified by the first stage, which is usually a low-gain amplifier. The output of the first stage is then fed into the second stage, which is a high-gain amplifier. The output signal from the second stage is the final amplified output.

3. What are the advantages of using a two-stage amplifier without feedback?

One advantage of using a two-stage amplifier without feedback is its simplicity in design and construction. This makes it cheaper and easier to manufacture. Additionally, without the feedback mechanism, this type of amplifier has a wider bandwidth and can handle larger input signals.

4. What are the disadvantages of a two-stage amplifier without feedback?

A major disadvantage of this type of amplifier is its lack of stability. Without feedback, it is more prone to oscillations and distortion in the output signal. It also has a lower gain compared to feedback amplifiers, which can limit its use in certain applications.

5. What are some common applications of a two-stage amplifier without feedback?

A two-stage amplifier without feedback is commonly used in audio equipment, such as guitar amplifiers and stereo systems. It is also used in RF and microwave systems, low-power communications, and instrumentation applications where simplicity and cost are important factors.

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