pre amplifier problem

[hamsiii]

Hello everyone,
I wanted to design a preamplifier for my project. It should amplify 20db in BJT stage (differential amp.) and 40db for opamp stage.
I think I have done the BJT and the buffer stage (first opamp) correctly but at the output of the last opamp (7th pin of opamp) i see a square wave and not amplified as 10 times (20db - 10 times).
Also my teacher says that BJT stage is not totally correct that I need to make some changes for 20db amplifying.
Thank you for your interest.

 

[ericgibbs]

hi,
Regarding your 3 OPA stages, you have a 12V split power supply, so the supply to the TL082 is +/-6V.
Look at this image, you can see that the maximum output swing is 6V-1.5V, ie
+/- 3.5V

Is the 10mV, 30KHz input signal 'peak' or 'peak to peak'..?

EDIT:
Sim in LTSpice assuming a 100mV signal after the BJT, this should give you a clue to the problem..

 

[hamsiii]

i have a single supply +12V for the project. We are forbidden to use -12V, so i created a virtual ground. And I can see why it becomes a square wave, as OPAMPs do not support that much voltage swing. I have 20mV pp input. I gave 200mVpp to first OPAMP stage and result is same as I need always 40db gain in OPAMPs.
So you say the circuit is working well? (This is a receiver amplifier and will only amplify the given "voice" signal.)

 

[ericgibbs]

i have a single supply +12V for the project. We are forbidden to use -12V, so i created a virtual ground. And I can see why it becomes a square wave, as OPAMPs do not support that much voltage swing. I have 20mV pp input. I gave 200mVpp to first OPAMP stage and result is same as I need always 40db gain in OPAMPs.
So you say the circuit is working well? (This is a receiver amplifier and will only amplify the given "voice" signal.)

hi,
The gain of 40dB, is as you may know *100, so if you input 0.2Vppk, the output of the final opa will 'try' to swing thru 20Vppk, which it cannot do as you have a split 12V supply and the limitations of the TL082 will only allow a +/- 4.5V.

This means that the top and bottom of the sine wave will be clipped/clamped at +/- 4.5V.... ie a flat top, like a square wave.

If you work back from '*100' with a 9V maximum output swing, this means that the first OPA should only have an input Vppk of 9/100 ie: 90mVppk.

OK.?

 

[hamsiii]

hi again,
my input is 0.02Vpp and as you said, it swings at last stage 20Vpp. But here is the problem, the voice signal coming from the antenna is about 20mVpp - 100Vpp and also BJT stage should have 20dB gain exactly. So, I could not have 90mVpp in the input of first OPamp.
I am thinking that clipped signal should not be that much of a problem, right?

 

[ericgibbs]

hi again,
my input is 0.02Vpp and as you said, it swings at last stage 20Vpp. But here is the problem, the voice signal coming from the antenna is about 20mVpp - 100Vpp and also BJT stage should have 20dB gain exactly. So, I could not have 90mVpp in the input of first OPamp.
I am thinking that clipped signal should not be that much of a problem, right?

hi,
The last thing you want to happen is clipping or distortion of an audio signal, it will sound awful.
If your incoming audio signal to the preamp is 20 >100mVpp and this is causing distortion of the signal output, then you have far too much gain.

At the moment you have an overall gain of 1000 with a limited output swing of 9Vppk, so thats 9V/1000 = 9mVppk permissible input to the preamp before it overloads.
Reduce the overall gain by a factor of 10.

 

[hamsiii]

hmm I understand, ok then at last stages of OPamps, instead of amplfying 10 times, i should choose variable resistors of 1-10k maybe. (the limitations of the projects are killing me ) So, I would have a 1-100x decrease in gain, which makes the swing in a good range which does not clip.
Thank you again Eric.

 

[ericgibbs]

hmm I understand, ok then at last stages of OPamps, instead of amplfying 10 times, i should choose variable resistors of 1-10k maybe. (the limitations of the projects are killing me ) So, I would have a 1-100x decrease in gain, which makes the swing in a good range which does not clip.
Thank you again Eric.

hi,
My advice would be to get rid of that BJT differential amp at the front end, its too fussy and it dosn't give you any advantage in the preamp.

The point to consider with the distortion in a preamp, is that the main amplifier after the preamp will further amplify that distortion...

EDIT:
Why not rewire that first TL082 opa, which is presently wired as a *-1, into a non inverting stage and this will provide a preamp with a high input impedance.???

 

[hamsiii]

No way, we should have that BJT stage and also the buffer stage of OPAMP (1st TL082). I know that distortion is increased as it is amplified and also I do not need 1000 times amplfying but these are the limitations that we are responsible for.

 

[ericgibbs]

No way, we should have that BJT stage and also the buffer stage of OPAMP (1st TL082). I know that distortion is increased as it is amplified and also I do not need 1000 times amplfying but these are the limitations that we are responsible for.

OK,
So if the rule is that you must use the BJT and OPA buffer, I would remove the last TL082 opa from the circuit.

BTW, your BJT amp needs more work, check the biassing and the 2nd base decoupling.

 

[hamsiii]

BTW, your BJT amp needs more work, check the biassing and the 2nd base decoupling.

Yes, I actually wanted to ask this, it amplifies 20dB but my teacher says it is not correctly. Would you mind helping me how I do that, if that's no waste of time for you?
I have done DC analysis as >> For current mirror: Assume Ibias = 0.4mA, Rbias= (Vcc-Vbe)/Ibias, Rbias=28k

and Ibias divides as Ic1=Ic2=0.2mA. Then Ib = 818uA, Rpi=31.7K, gm=7.7mS,
Av=10=gm*RL/(2*(1+gm*Rc)) Is this analysis accurate?

 

[hamsiii]

Or let me ask you one more thing. How do I eliminate the DC value of the first stage output as it should swing around 0V?

 

[ericgibbs]

Or let me ask you one more thing. How do I eliminate the DC value of the first stage output as it should swing around 0V?

Consider the effect of C8, the 47nF capacitor.????

 

[hamsiii]

OK, I redesigned the circuit, get rid of second OPAMP and understand how coupling capacitors eliminate DC.
But this time, the first stage (BJT pairs) do not give 20dB gain. I am totally confused. May anyone give me a good source on BJT differential amp. (both for DC and AC) analysis?

 

[ericgibbs]

OK, I redesigned the circuit, get rid of second OPAMP and understand how coupling capacitors eliminate DC.
But this time, the first stage (BJT pairs) do not give 20dB gain. I am totally confused. May anyone give me a good source on BJT differential amp. (both for DC and AC) analysis?

hi,
Look at these links..
bjt differential amplifier tutorial - Google Search

How is the Base of the rightside BJT of your circuit connected.???? regarding an AC signal.???

 

[audioguru]

Why are you using a differential transistor circuit when your input signal is not differential?

 

[ericgibbs]

Why are you using a differential transistor circuit when your input signal is not differential?

He has told me that is the rule set for the Homework exercise..
I suggested he drop the BJT, but he is not allowed too.

 

[hamsiii]

base of the right side BJT is connected to ground as I cannot give 2 different inputs to the circuit. The voice which comes from the antenna is the signal only.
If I give the same input to Bases of BJTs , I cannot create a differentail pair.

 

[audioguru]

A differential circuit is used with a differential input signal which causes one input to go positive when the other input goes negative.
But you don't have a differential signal so you don't need a differential circuit. The teacher should have provided a differential signal.

Your required circuit had too much gain. All radios have too much gain then have a volume control. Your circuit was missing a volume control.

 

[hamsiii]

I understand you completely, but these are the requirements. Thank you again for your interest, Eric and audioguru.
I will use some of the resistors as variable resistors so they will be volume pots.