Question 22.7: Simulate the CMRR of the diff-amp in Fig. 22.12. Show that t......
Simulate the CMRR of the diff-amp in Fig. 22.12. Show that the CMRR falls with increasing frequency.
As mentioned above, the (low-frequency) calculated CMRR is 86 dB. To simulate the CMRR, we perform AC simulations on two of the diff-amps in Fig. 22.12 so we can simultaneously determine A_d and A_c. The ratio of these gains is the CMRR. Figure 22.16 shows the simulation results. The common-mode gain, A_c , depends on the DC common-mode voltage. The results seen in Fig. 22.16 are with a common-mode (DC) voltage of 700 mV. Dropping the common-mode voltage down to 500 mV causes the low-frequency CMRR to drop down to 50 dB. The reason for this is that, with higher DC common-mode voltages, the voltage across the tail current source is higher, so its output resistance is larger.
