Create a two-stage amplifier without feedback

[bergen89]

< 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.

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?

 

[NascentOxygen]

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.

 

[bergen89]

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.

 

[NascentOxygen]

Do you know the purpose of resistor RE18?

 

[bergen89]

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).

 

[NascentOxygen]

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.

 

[Svein]

Take a look at this paper: http://www.linearaudio.nl/linearaudio.nl/images/pdf/otala%20low%20tim%20amp.pdf
It is a detailed description of a very famous power amplifier and the included preamplifier.

 

[Svein]

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There are hundreds of circuits on the web - most of them either outdated or specialized.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​

 

[bergen89]

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​

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.

 

[Svein]

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?

 

[NascentOxygen]

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?

 

[bergen89]

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.

 

[bergen89]

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?

 

[NascentOxygen]

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.