Download The Two-Stage Op-Amp Input Common

The Two-Stage Op-Amp
Figure shows the basic two stage op-amp made using an NMOS diff-amp and a PMOS commonsource amplifier (M7). As seen in Fig. M7 is biased to have the same current as M3 and M4. Note
also the addition of the compensating network consisting of a compensation capacitor, Cc, (Miller
compensation) and a zero-nulling resistor R2. Because the op-amp doesn't have an output buffer,
it is limited to driving capacitive loads and very large resistances (comparable to the output
resistance of a MOSFET, that is, megaohms).
Low-Frequency, Open Loop Gain, AOLDC
The low-frequency, open loop gain of the op-amp is calculated as the product of each stage gain,
that is,
where the output resistance of the cascode current source load, M8, is assumed to be much larger
than the M7 output resistance, rop. we get an AOLDCofM2V/V.
Input Common-Mode Range
The minimum input common-mode voltage is given by Eq. or 450 mV. The maximum input
common-mode voltage is given by Eq. or 930 mV. This means, for proper operation of our twostage op-amp, the input voltages (v and vm) should fall within the range of 450 to 930 mV. If they
go outside this range, the op-amp gain drops, and it is likely that the circuit employing the op-amp
will not function properly. Figure shows a SPICE DC sweep where the inverting input (vm) is held
at 500 mV and the noninverting input (v ) is swept from 495 to 505 mV. The slope of this transfer
curve is the DC open-loop gain of the op-amp, A0LDC.
Basic two-stage op-amp.