Two Stage Op Amps

One stage op amps allow the small-signal current produced by the input pair to flow directly through the output impedance. The gain is limited to the product of the input pair transconductance and the ouput impedance.

In some applications the gain and/or the output swings provided by cascade op amps are not adequate. In such cases, a two stage op amp is needed where the first stage provides high gain and the second, large swings. In short, it isolates the gain and swing requirements.

In the above figure, each stage can incorporate various amplifier topologies but the second stage is typically configured as a simple common-source stage so as to allow maximum output swings.

The following figure shows an example where the first and second stages exhibit gains equal to gm1,2 (ro1,2 || ro3,4)  and gm5,6 (ro5,6 || ro7,8 )  respectively. The overall gain is therefore comparable with that of a cascode op amp, but the swing at Vout1 and Vout2 is equal to VDD-|VOD5,6 |-VOD7,8.

To obtain a higher gain, the first stage can incorporate cascode devices, as depicted in the following figure. With a gain, say 10, in the output stage, the voltage swings at X and Y are quite small, allowing optimization of M1-M8 for higher gain. The overall voltage can be expressed as

A two stage op amp may provide a single-ended output. One method is to convert the differential currents of the two output stages to a single-ended voltage. As illustrated in the following figure, this approach maintains the differential nature of the first stage, using only the current mirror M7-M8 to generate a single-ended output. If the gate of M1 is shorted to Vout2 to form a unity-gain buffer, then the minimum allowable output level is equal to VGS1+ VISS, severely limiting the output swing.

Frequency Compensation:

Typically op amp circuits contain many poles. In a folded-cascode topology, both the folding node and the output node contribute poles. For this reason, op amps must usually be compensated, i.e. their open-loop transfer function must be modified such that the closed-loop circuit is stable and the time response is well-behaved.

The first approach requires that we attempt to minimize the number of poles in the signal path by proper design. Since each additional stage contributes at least one pole, this means the number of stages must be minimized, a remedy that yields low voltage gain and/or limited output swings.

The second approach, on the other hand, retains the low-frequency gain and the output swings but it reduces the bandwidth by forcing the gain to fall at lower frequencies.