Experiment 9- Single Stage Amplifiers with Passive Loads - MOS

UC Berkeley EE 105

1.0 Objective

This is the second part of the single stage amplifier lab. We will be dealing with MOS amplifiers in this experiment.

To show your understanding of the lab, your write-up should contain:

A table showing the input resistance, output resistance, and gain

A discussion on trade-offs issues among the three parameters A_v, R_in, and R_out

A discussion explaining the advantages and disadvantages of the different amplifiers

A discussion of the differences between similar MOS and BJT amplifier stages

2.0 Prelab

H&S: Chapter 8.3, 8.9

You will now consider biasing issues with FETs. Below is an NMOS transistor that will be configured as a Common Source Amplifier. For the figure below, bias the circuit so that V_OUT = 2.50 V. Determine the proper bias voltage V_BIAS needed to achieve this. What can you say about the value of V_BIAS when compared to V_BIAS for the common emitter amplifier? Use the following MOS parameters for hand calculation and for SPICE, in which you should plot V_OUT vs. V_BIAS.

V_TO = 0.9 V, K_p = 20 x 10^-6 A/V², = 0.05 V^-1

FIGURE 1.

3.0 Procedure

FIGURE 2.

Many of the amplifiers will contain special biasing current sources to set the collector currents of the npns or the drain currents of the FETs. The drain or collector currents will be equal to the current in an external resistor R_BIAS. The user provides R_BIAS across pin 28 and pin 22. The current through the resistor is equal to I_BIAS which sets the I_D for the NMOS transistor. I_BIAS can be found by use of a voltmeter across R_BIAS.

3.1 Common Source Amplifier

3. Figure 3 shows a Common Source amplifier. Let R_D = 50 k

4. Let v_in be a sinusoid with an amplitude of 100 mV at a frequency of 5 kHz.

5. Measure V_OUT and verify that the transistor is operating in the constant-current (saturation) region. Measure the value of the drain current and compare it with the calculated value.

FIGURE 3.

6. Use the oscilloscope to measure the voltage gain v_out/v_in. Make sure that the output isn't clipping. Also measure v_g/v_in. Find the gain. Compare the value of the gain to that of the Common Emitter.

7. One major difference between bipolar and MOS transistors is that the MOS transistor has an infinite input impedance. Because of this high input impedance, there is no voltage attenuation from the voltage source to the amplifier input, even if the voltage source has a large source resistance. Verify that this is true by measuring v_g/v_in. You can obtain an estimate of the input resistance of the common source amplifier from the gain v_g/v_in.

8. Measure the output impedance using a technique similar to the method used to measure the output impedance of the bipolar amplifiers in Exp. 8.

3.2 Common Drain Amplifier (Source Follower)

1. Figure 4 shows a Common Drain amplifier with a current source biasing. The current source is really an NMOS transistor and its small-signal resistance is 1 / (_nI_D). Its bias current is the current through R_BIAS = 10 k. Let v_IN be a sinusoid with an amplitude of 200 mV at a frequency of 5 kHz and a DC offset of 3 V. Repeat the steps above to find the gain and output resistance. As with common collectors, common drains are also used as voltage buffers. I_BIAS is the same current through R_BIAS.

FIGURE 4.

4.0 Optional Experiments

4.1 Common Source with Source Degeneration

1. Connect the circuit of figure 9. Let R_D=5 k, R_S=500 , and V_BIAS=3 V. Find the gain, input resistance and output resistance. Note: V_Tn will not equal V_TOn because of the backgate effect. Source degeneration in MOS amplifier stages is not as widely used as emitter degeneration in bipolar circuits. The transconductance of MOS transistors is much lower than that of bipolar transistors so that further reduction in g_m is usually undesirable. Also, the beneficial effect of raising the input impedance of a bipolar transistor is irrelevant for MOS transistors since the input impedance is infinite. Degeneration is occasionally useful for making the transconductance independent of the device characteristics.

FIGURE 5.