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Can I wire opamp like this and expect it to work?

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vlad777

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Can I wire opamp like this and expect it to work?

Expected gain 1..1000.

Audioguru's schematic use too much caps.

...sorry output cap is 1000 uF
 

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audioguru's schematic has that many capacitors for a reason.

The capacitor values depend on the minimum frequency of interest:

[latex] C = \frac{1}{2 \pi Fc \times R}[/latex]

R = the resistor in series with the capacitor.
Fc = the lower cut-off frequency; it's a good idea to make this half the lowest frequency of interest.

C = the required capacitance in Farads.
 
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The amplifier has many problems; most due to the poor performance of the 741. At such a high gain, the 741's input offset will be amplified by 1000, and will likely rail its output. You will have to connect the 10k offset nulling pot between pins 1 and 5 as shown in **broken link removed**. I helped the offset problem by redoing the way you biased the inputs at 1/2 of 9V. Note that I raised the value of the voltage divider resistors (to minimize battery drain), and I made it so that both the inverting and non-inverting inputs see a source resistance of 11K, which will minimize the offset due to the input bias current of 500nA. I added the 22uF bypass to keep the center point of the voltage divider from being driven from the input signal.

The poor Gain-Bandwidth product of the 741 shows up in a frequency plot. Note that the gain is 1000 (60db) up to ~200Hz, at which point the closed-loop gain begins rolling off. The whimpy input coupling capacitor of 1uF causes the gain to fall off on the low end, as does the output coupling cap (if the output is terminated by a 10K resistor...).
 

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Thanks everyone for answering :)

MikeMl I understand most of what you did but what do you recommend
for input and output caps values. Is bigger better?
Is there a better option instead of 741 for better gain bandwidth plot
and what is the most usual?
 
Hero999, the only cap that is clearly in series with any R (in all four schematics) is the one in feedback (gain regulation) resistors.
Do you mean that one?

Edit: I was newer good with 2*Pi*f ..s :)
 
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TL071 single, TL072 dual and TL074 quad opamps are much better than a lousy old 741 but cost almost the same. Their output produces frequencies at full output to 100kHz but a 741 opamp has trouble above only 9kHz. We can hear to 20kHz. Their high frequency gain is much higher but still might not be high enough.

A voltage gain as high as 60dB is 1000 times. Why do you need gain to be so high?
 
. Why do you need gain to be so high?

Don't know if I really need it to be so high. I am trying to catch and record on my comp
nervous signals like leg muscule contraction or hart beats.
 
Thanks everyone for answering :)

MikeMl I understand most of what you did but what do you recommend
for input and output caps values. Is bigger better?
Is there a better option instead of 741 for better gain bandwidth plot
and what is the most usual?

What aspect of the amplifier performance do you want to improve? If you truly need a gain of 1000, cascading two amplifiers, each with a gain of ~31 will result in a much better product. That will improve both the high-frequency performance, as well as making the offset more manageable.

If you can live with the high-frequency roll off, then increasing the input and output coupling caps to 10uF or 22uF will lower the low frequency roll off to ~20Hz.

If I were doing this, I would use a modern CMOS rail-to-rail opamp like the TLV2371 (TLV2372 has two per package). It has a GBW of 3MHz and much improved specs compared to a 741. I would use two gain of 32 stages, as shown below:

Trying to get the full gain of 1000 out of one high-speed opamp is problematic because of stability and stray capacitance issues.

-3db bandwidth with the two stage circuit is 3Hz to 80kHz.
 

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A gain of 100 is plenty to hear a heartbeat from an electret mic in a shallow cup made from a jar lid. But if you want to hear a heartbeat 10meters away or more then you need lots of gain, maybe 1000.
 
A gain of 100 is plenty to hear a heartbeat from an electret mic in a shallow cup made from a jar lid. But if you want to hear a heartbeat 10meters away or more then you need lots of gain, maybe 1000.

Didn't really think about microphones but skin electrodes, but good to know.
BTW how much gain do the cell phone mic has?

I heard stuf like it can hear the conversation about 20 meters away or more
if you have the right software on right machine.
 
Skin electrodes are used with an electro-cardiogram (ECG), not an electronic stethoscope.

Cell phones, answering machines and kids toys use excellent but inexpensive electret microphones. They don't have any gain. Their preamp circuit has as much gain as you need. A parabolic dish can focus sounds into an electret mic when the sounds are very far away.
 
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