Curtis Mayberry EE 435

Lab 3: Common Source Amplifier

Introduction In this lab a common source amplifier is designed and then cascoded to meet a more stringent gain

requirement. The requirements for the common source amplifier are:

Vdd=-Vss=2.5V

Total Power < 0.3mW

DC Gain>40dB

Gain Bandwidth Product>45MHz

Load Capacitor: 2pF

Slew Rate>25V/us

Output Swing Range: Vss+50mV to Vdd The cascoded common source amplifier has the same requirements except it requires a DC gain greater than 60 dB.

Design I started the design of the common source amplifier by choosing the bias current according to the

desired slew rate.

Next the excess bias of the transistors was chosen according to the gain of the amplifier.

Finally the devices were sized:

The size of M1 was adjusted to 40µ

The biasing of the circuit was chosen using the auto-biasing circuit shown in figure 1. These bias

voltages were then used to bias the circuit shown in figure 2.

Figure 1: Auto-bias generating circuit

Figure 2: DC biased CS amplifier circuit used to measure the gain and phase

Results The measurement of each of the requirements is outlined below.

Power The total power consumed is 0.256 mW. IDQ was measured when determining the DC operating point.

DC Gain The DC gain of my common source amplifier design is 41.4dB as shown in figure 3 below.

Figure 3: Gain plot showing the DC gain, bandwidth, and unity gain frequency (GB product)

Gain Bandwidth Product The gain bandwidth product was found to be 54.79 MHz. The GB product is approximately the unity

gain frequency which is shown in figure 3 above. The bandwidth was found to be 454.6 kHz.

Slew Rate Next the circuit shown in figure 4 was used to find the slew rate. The slew rate was found by inputting a

step function and then measuring the slope in the output at the rising edge. The step function ranged

from -1v to 1v with a rise and fall time of 1ns and a period of 100µs. A transient analysis as shown in

figure 5 was used to measure the output’s derivative.

Figure 4: Step input circuit to find the slew rate

Figure 5: Slew rate plot from a step input (Finding the slope of the output change)

Output Swing Range The output swing range was also found using a transient analysis and inputting a sinusoidal input instead

of the unit step used to measure the slew rate. The output range was found to be -2.453v to 2.498v

(≈2.5v). The Output swing range can be seen in figure 6.

Figure 6: Output Swing Range found using a transient analysis

Gain and Phase Plots Figure 3 is the gain response and figure 7 is the phase response as the input frequency was swept from 1

Hz to 1GHz.

Figure 7: Phase frequency response plot

DC Transfer Function The DC transfer function plot of Vo vs. Vin is shown in figure 8.

Figure 8: DC Transfer Function Plot

Improving DC gain performance In an effort to improve the DC gain performance of the previously designed common source amplifier

the CS amplifier can be cascoded. The cascaded amplifier is able to increase the gain of the system by

20-30 dB which can help to meet the increased gain requirement of > 60dB

Cascoded CS Results I proceeded to cascode the design and included the auto-biasing circuit shown in figure 9. The sizes

choosen started as the same used in the non-cascoded version. However the slew rate of the circuit was

not able to meet the requirements. I then proceeded to increase the current to 57 µA since I had slack

in my power requirement and used this extra power budget to increase the slew rate.

The sizes for M1 and M2 were set to 32 and 95 respectively

Figure 9: Auto-biasing cascoded common source amplifier

DC Gain The DC gain of the cascaded CS shown in figure 9 is 79.6 dB as shown in figure 10.

Figure 10: The gain frequency response showing the

Frequency Response The gain plot of the frequency response is shown in figure 10 and the phase plot is shown in figure 11.

Figure 11: cascoded CS phase response

Power The power consumed is 0.285 mW

Gain-Bandwidth Product The Gain-bandwidth product is 45.37 MHz as shown by the unity gain frequency in figure 10.

Slew Rate The slew rate of the cascoded amplifier is 28 v/µs as shown in figure 12.

Figure 12: Transient analysis to find the slew rate

Output Swing The output swing was between -2.05v and 2.1v as shown in figure 13. A decrease in the output swing

can be expected due to the extra transistors added in order to cascode the circuit, which reduces it by at

least an extra excess bias.

Figure 13: Output swing range