Sunday, April 06, 2014

IC Design Interview 6: Op-amp Circuits

You must know your op-amp circuits to do analog circuit design! During an interview, you even have to derive the operation equations. Each of these derivations starts with the result in bold (because you must just know that) followed by the derivation.

Inverting Op-Amp

Vout = -(R2/R1)Vin

Vin/R1 = -Vout/R2
-(R2/R1)Vin = Vout

Non-Inverting Op-Amp (Buffer)

Vout = Vin(1+R1/R2) = Vin((R2+R1)/R2)

V- = Vout R2/(R2+R1)  [Voltage Divider]
Assume V- = Vin
Vin = Vout R2/(R2+R1)
(R2+R1)/RVin = Vout
The special case of R2 being open (infinite resistance) creates a Unity Gain Buffer

Difference Amplifier

Vout = R2/R1 (VB-VA)

Assume V- = V+
V+ = VB R2/(R1+R2)   [Voltage Divider]
Since I1=I2                 [Kirchoff's Current Law]
(Vout-V-)/R2 = (V--VA)/R1
Solve for V-
R1Vout+R2VA = V-
Which is the same as V+, so
R1Vout+R2VA = VB R2
      R2+R1             R1+R2
Which can be solved for Vout
                                                                                         Vout = R2/R1 (VB-VA)