Two Stage Differential Op-Amp

201

03:01:32

A simple amplifier designed to take a signal from a microphone/instrument. Gain is ~1200 (60 dB). 

The circuit has two switches on the far left The top most switch will route the input signal to both inputs of the op-amp (common mode), with the switch off it is set in differential mode.

The bottom switch chooses which input to use on the op amp.

This is an audio amplifier, specifications are as follows:

1. Gain of 1228 (~61.7 dB)
2. CMMR of 50 dB between 40Hz and 20kHz
3. Input impedance between 1-10k (with one input grounded)
4. Frequency Response within +- 0.5 dB from 40Hz to 20kHz.

The major problem in the design here is getting the correct gain for the non inverting input on the first stage, while keeping the input impedances in the correct range. The 2k resistor on top left most corner is the major factor here, too high and you lose gain, too low and you lose input impedance.

The second stage is just a simple non inverting amplifier that has a switch that drops the signal by -20 dBu.

published 10 years ago

hurz

10 years ago

Something wrong at input, why is it short circuit? For test?

hurz

10 years ago

http://everycircuit.com/circuit/6191879023493120

blaxzen

10 years ago

The switch is so that both the inputs of the amplifier receive the same signal, the reason is to test the CMMR, how well the op-amp will reject signals that are the same on both inputs. The op-amp is differential, it will apply gain to any difference between the inverting and non-inverting input voltage (differential voltage)

hurz

10 years ago

If the voltage is equal, then the output for everycircuit's perfect opamp's is 0! So the voltage is not equal!

blaxzen

10 years ago

Thats the idea :), you use a balanced input like an XLR, and then any difference in the wires gets amplified, whereas any noise picked up on both the wires will be removed by the amplifier.

hurz

10 years ago

Yes

arunwarrior

10 years ago

one doubt pls reply why compensating resistor is not needed in a differential op amp?

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