op-amp config

[audioguru]

Can I done it by cascading just 2 similar configs I showed in my first post?

The opamps in your first post have no reference voltage (0V) on the pin 3 input of the opamps so they do not work.
I don't know what is the DC voltage at the output of your powered electret mics.
If you cascade two opamps that each have a gain of 101 then the total gain is 10,201 which is much too high.
If you cascade amplifiers each with a gain of 471 then the total gain is 221,841 which is ridiculous.

 

[zesla]

The opamps in your first post have no reference voltage (0V) on the pin 3 input of the opamps so they do not work.

Do I need to have a reference voltage on non inverting pin (a voltage divider)? What's the reason? Is that due to this fact that inverting pin is not able to provide the DC reference point because it has a capacitor in series?

I don't know what is the DC voltage at the output of your powered electret mics.

My electret Mike will not have any DC because it should has a coupling cap.

If you cascade two opamps that each have a gain of 101 then the total gain is 10,201 which is much too high.
If you cascade amplifiers each with a gain of 471 then the total gain is 221,841 which is ridiculous.

No I want each op-amp to provide a gain of 30 MAX (900 for the whole). I want to be able to change the gain and reduce too.

Can you show me a circuit doing so by 2 stages too? I want the second stage to be something like what I showed at my first post.

Thanks

 

[zesla]

Here's what I designed by myself to do so.
Please tell me your ideas about it.

Thanks

P.S I need R8 & R14 (I use a pot for them) to provide some of DC Offset for the circuit, but do not know if I am able to put that pot to the input of the first stage somehow too, so that I prevent it to eat too much of current (and maybe being fried)?

 

[audioguru]

You can replace R1 with a piece of wire since it acts like an attenuator.
The input biasing resistors can be 10 times higher or more so that C2 and C4 can have a much lower values since a noninverting opamp has a very high input resistance.

Your first opamp has its input DC reference voltage at 0V so it needs a dual-polarity supply.
The second opamp has its input biased to a positive voltage so it might work with a single-polarity supply.
With a single-polarity supply, bias the input of the opamp at half the supply voltage since it has a DC gain of 1 and the opamp is symmetrical.

 

[zesla]

The input biasing resistors can be 10 times higher or more so that C2 and C4 can have a much lower values since a noninverting opamp has a very high input resistance.

But What about R14 R15 and R8? They cause the input iimpedances to not be very high?!

Your first opamp has its input DC reference voltage at 0V so it needs a dual-polarity supply.
The second opamp has its input biased to a positive voltage so it might work with a single-polarity supply.
With a single-polarity supply, bias the input of the opamp at half the supply voltage since it has a DC gain of 1 and the opamp is symmetrical.

My power supply is a dual one for both op-amps. But As I told for some reason I need some of DC offset for my circuit.

What Is your idea about C3? Do I need to use it?

And the final question, As both op-amps are used as non inverting config so please What's the input impedance for each op-amp at the said circuit?

Thanks like always

 

[audioguru]

But What about R14 R15 and R8? They cause the input impedances to not be very high?!

Yes.
Look at the datasheet for the TL072 dual opamp. Its inputs are Jfets with an extremely low input current that is typically 65pA and an input resistance of typically 10 million-million ohms. So R14 can be 2M and R8 can be 8M if you want. Then C4 can be 1nf. R15 can be 10M then C2 can be 1nF.

My power supply is a dual one for both op-amps. But As I told for some reason I need some of DC offset for my circuit.

Then the second opamp might not work because its input voltages might be out of the allowed common-mode voltage range for the TL072.

What Is your idea about C3? Do I need to use it?

Maybe. It blocks DC so that the first opamp has a DC gain of 1 and its output idles at the same voltage as pin 3 input which is 0V.
The opamp has a max input offset voltage of 10mV which would be amplified to plus or minus 0.28V if C3 is not used. C3 also reduces frequencies below 16Hz.

And the final question, As both op-amps are used as non inverting config so please What's the input impedance for each op-amp at the said circuit?

Since the TL072 opamp has such an extremely high input resistance then the input impedance of the first opamp is R15 and the input impedance of the second opamp is R14 in parallel with R8.

 

[zesla]

Since the TL072 opamp has such an extremely high input resistance then the input impedance of the first opamp is R15 and the input impedance of the second opamp is R14 in parallel with R8.QUOTE]

Ok thanks.
But I have another question.
As you told the input impedance of a op-amp is extremely high, So please tell me why we need to limit that by one or two resistors? Anyway the more input impedance the better result we will get, right? If so why to limit it?

 

[audioguru]

Very few signal sources need an amplifier with an extremely high input impedance. A high input impedance allows the value of the coupling capacitor to be less then it is smaller and a high quality lower price (because of the lower value) film capacitor can be used.

An opamp will not work properly if its input does not have a suitable DC reference voltage.
Your first opamp has its DC input reference voltage set to 0V by R15. Your second opamp has its DC input reference voltage set with two resistors as a voltage divider.

 

[zesla]

OK I just used AD826 high Speed op-Amp which has almost 350v/us of sllewrate, and seems to be able to give me a gain of more that 1000 at audio spectrum (20kHz MAX), righ audioguru? So I want to use just one op-amp config for my task.

If you take a look at the below pic you can see that R5 is 1K and C1 is 22uF.
But there is a problem when I perform the circuit. there is a delay befor the circuit does work due to the C1 value (i.e 22uF). I can reduce it to say 1uF but I have to use a larger resistor (say 20k) but this time to cause the gain to be constant at almost 500 I have to use a feedback resistor of almost 10M, which I am not sure if it is reasonable to use a such resistor?


Any idea would be appreciated

 

[audioguru]

The AD826 has an input bias current that is too high to use a feedback resistor value that is higher than about 50k ohms.
With an input bias current at its max of 6.6uA then your 500k feedback resistor causes the output DC voltage to be shifted 3.3V.
Its input resistance is fairly low at 300k ohms which will reduce gain when the feedback resistance is high.

Your 22uF capacitor is charged by the 500k plus 1k resistor in about 55 seconds. But the very high value capacitor produces an output that is -3dB at 7.3hz and is flat down to about 22Hz.

 

[zesla]

Thanks a lot,

So what to do please?

 

[audioguru]

Thanks a lot,

So what to do please?

If you need lots of gain and low noise at audio frequencies then use a dual or quad low noise audio opamp like a TL072, TL074 or a OPA2134, OPA4134. Use a few stages that have adequate bandwidth and let their gains multiply.

 

[zesla]

If you need lots of gain and low noise at audio frequencies then use a dual or quad low noise audio opamp like a TL072, TL074 or a OPA2134, OPA4134. Use a few stages that have adequate bandwidth and let their gains multiply.

Don't you have any solution regarding just one Op-amp? Speacially for AD826 please?

 

[audioguru]

An AD826 high frequency opamp with a 47k feedback resistor, a 47 resistor in series with a 220uF capacitor to ground will have a voltage gain of 1000, a fairly high noise level (hiss) and a bandwidth from 20Hz to about 70kHz. The 220uF capacitor will take almost one minute to charge.

 

[zesla]

An AD826 high frequency opamp with a 47k feedback resistor, a 47 resistor in series with a 220uF capacitor to ground will have a voltage gain of 1000, a fairly high noise level (hiss) and a bandwidth from 20Hz to about 70kHz. The 220uF capacitor will take almost one minute to charge.

One Minute is too very long. I do'nt like the Hiss as well too.
Well I do not know the exact config for 2 stages op-amp amplifier based upon two TL072 fot this task?
I just tried it once but the result was not good.
I need realy help from experts here to do a 2 stages amplifier with the gain of more than 500.

 

[audioguru]

Each opamp in a TL072 dual opamp will have a gain of only 22.4 for a total gain of 500. The noise and distortion will be very low and the bandwidth will be to about 50kHz.

 

[zesla]

Each opamp in a TL072 dual opamp will have a gain of only 22.4 for a total gain of 500. The noise and distortion will be very low and the bandwidth will be to about 50kHz.

Yea But I have a problem to design and make it. I am not a designer so just need help please.

 

[zesla]

I even do not mind if a transistor befor my op-amp config can be helpful to generate some of gain.

 

[audioguru]

I even do not mind if a transistor befor my op-amp config can be helpful to generate some of gain.

A single transistor can have distortion as high as 40%.
An opamp has distortion as low as 0.00008%. A huge difference.

 

[zesla]

A single transistor can have distortion as high as 40%.
An opamp has distortion as low as 0.00008%. A huge difference.

Ok so please help me to convert this circuit so that It does work. I guess there is a problem somewhere in the circuit! I guess the problem is on the first stage because the circuit did work when I just used the second stage by itself.

Thanks