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DiscreteTransistor AC Op-AMP Circuit

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This circuit is an inverting descrete transistor ac op-amp.  It doesn't pass DC signals.  The gain is set by the feedback resistor R2 and input resistor R1.  The transistor amp is a very high gain amplifier.  D1 is a bias Diode.  So the base of Q2, and collector of Q3 is biased at about 4V. The low pass filter R4 and C3 is a compensation network to stop oscillations.  C2 and C1 block DC and pass AC through the circuit. 

Since the the circuit has such high gain, the base of Q2 looks like a virtual AC ground, because Vout/infiity= 0V.  Thus, any AC current flowing in R1 also flows through R2. 

IR2= Vin/R1. 

Vout/R2= -IR2.

Thus

Vout/Vin = -R2/R1

Check out our Op-Amp Resistor-Calculator.

Equations:

Vout/Vin = -(R2/R1)

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