A directly coupled differential amplifier has a differential voltage gain of A_d

Question

A directly coupled differential amplifier has a differential voltage gain of A_d = 240 V/V, an input resistance of R_id = 1 M Ohm, an input bias current of I_B = 50 nA, a maximum offset current magnitude of I_os = 4 nA, and maximum offset voltage magnitude of V_os = 1 mV Calculate the worst case output voltage if the amplifier input terminals are connected to ground via 100 k Ohm resistors Repeat if the noninverting input terminal is grounded directly and the inverting input is grounded via a 100 k Ohm resistor

Explanation / Answer

1)Let Vt be the worst case output voltage

Vt = Ad*(Vos) + 100K*(Ios)    

(Since both amplifier input terminals are terminated through 100 K ohm resistor only offset current multipled by resistance will be reflcted in output voltage due to current)

Vt = 240*(1 mv) + 100K*(4 nA)  

Vt = 0.24 V + 0.4 mV

Vt = 0.2404 V                                        .......................1

This is the required worst case output voltage

2)

Let VT be the worst case output voltage

VT = Ad*(Vos) + 100K*(IB)    

(Since only inverting input is terminated through 100 K ohm resistor therefore input bias currentt multipled by resistance will be reflcted in output voltage due to current)

Vt = 240*(1 mv) + 100K*(50 nA)  

Vt = 0.24 V + 5 mV

Vt = 0.245 V                                        .......................2

This is the required worst case output voltage

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