Differential amplifier layout

Figure 1. Schematic diagram for Differential Amplifier Differential Amplifier consists of a differential pair and current mirrors. In this circuit, Mn2 and Mn1 make the differential pair and Mn8 and Mn5 and the mosfets Mp3 and Mp4 make the current mirrors. Differential amplifiers produce an output with a difference of two voltages. The main advantage of this circuit is that it is more immune to noise than an o rdinary amplifier.

Fig 2. Modified Specifications of the circuit The circuit in figure 1 consists of transistors that have mismatches. Ratios of current mirror are around 10 while the ratio of differential pair is around 5. To modify this circuit, we apply common centroid technique. To better match the transistors, we add multipliers.

Fig 3. Differential pair layout with common centroid technique and twisted connections Twisted connections improve the layout by minimizing the noise through canceling out electromagnetic interference.

Fig 4. NMOS Current Mirror Similar to Fig 3, this layout also applies the twisted pair connections and also minimizing noise.

Fig 5. PMOS current mirror The technique applied in this layout is the same with the figure 4 and figure 3 which is the common centroid. However, this one is different for making the overall layout to shape like a square.

Fig 6 Overall layout of the differential amplifier DRC and LVS verification:

Fig 7 DRC Verification

Fig 8 LVS verification

RESULTS and DISCUSSIONS

Fig 9. Presim Gain of the differential amplifier with 43.1dB

Fig 10 Presim phase margin of the differential amplifier with 84.9 degrees

Fig 11. Differential amplifier gain with 43.1dB

Fig 12. Post sim phase margin around 84.4 degrees

Gain Phase Margin

Presim 43.1dB 84.9 degrees Table 1.1 Comparisonn of Presim and Postsim

Postsim 43.1dB 84.4 degrees

Conclusion:

Normal layouts can be improved by applying techniques. As we can see from Table1.1, Presim data is so much improved when we layout it. This is because we use the technique common centroid layout. Applying this technique can reduce the area and conserve more space for additional circuits in a wafer. Also, applying twisted pair connects immunes the circuit from noise. Normal connections has a high chance of not improving the pre simulation that is why applying these type of techniques help a lot in making the stability and the amplification of the differential amplifier. We can see that the stability of the differential amplifier in the layout is improved compared to the pre simulation. However, this can further improved by adding compensating devices like capacitors and resistors.