Single-Stage MOSFET Amplifiers

Transcription

1 ECE SingleStage MOSFET Amplifiers Lab 8 SingleStage MOSFET Amplifiers n this lab we will investigate the gain properties of a sourcefollower and a commonsource amplifier. 8.A. PreLab Calculations All calculations go in your notebook. No writing equations on a piece of paper and then copying them down in your notebook. Create tables for the two amplifiers discussed in this lab. Your prelab consists of the data contained in the tables and the calculations that support those values. Note that the current source can have a value between.5 and 1 ma. Use these min and max values in your calculations. 8.A.1. CommonSource Amplifier We will use the current source of the previous lab to bias the commonsource amplifier below: Rc 7.5k C6 1U ZN336A RL out.1u 1u in MPF12 CE 47u.1u 1u Obtain a data sheet for the ZN336A transistor. Make the following calculations using the maximum and minimum current for the current source, resistor tolerances, Tmin =.8 volts, Tmax = 2.4 volts, and K min = 15 ma/ 2 and K max = 3 ma/ 2 Do not use PSpice to make these calculations. Maximum and minimum values of the bias drain voltage ( D ). 81

2 ECE 25Lab8: SingleStage MOSFET Amplifiers Maximum and minimum values of the bias source voltage ( S ). Maximum and minimum values of the gain O / N if RL = 1 M. Maximum and minimum values of the gain O / N if RL = 7.5 k. Create a table for prelab and laboratory results and record these values in the table. You may be chasing your tail for a long time while trying to figure out how to choose between using Tmin, Tmax, K min, and K max. f you remember, we have the following equations for the commonsource amplifier: D gm vo v in K 2K( g m GS GS ( R L T R T C 2 ) ) The difficult equation to calculate with minimum and maximum values is the transconductance equation because K, GS, and T all have maximum and minimum values, and we are not sure how to plug them in to get maximum and minimum values for g m. We can avoid this problem by solving the top equation for GS T and then substituting the result into the center equation: D K 2 there fore GS T GS T D K Substituting this into our transconductance equation gives gm 2K( GS T ) 2K D K which can be rewritten as g 2 K m D n this form it is easy to calculate max and min values of g m because, for our circuit, D is determined by the current source and K is determined by the MOSFET. Even though K has max and min values, and D has max and min values, they are both completely independent of one another and can be easily chosen so maximize or minimize g m. 8.A.2. SourceFollower Amplifier We will use the current source of the previous lab to bias the sourcefollower amplifier below: 82

3 ECE 25Lab8: SingleStage MOSFET Amplifiers.1u 1u ZN336A in Cs 47u MPF12 RL out.1u 1u Make the following calculations using the maximum and minimum current for the current source, resistor tolerances, Tmin =.8 volts, Tmax = 2.4 volts, and K min = 15 ma/ 2 and K max = 3 ma/ 2 Do not use PSpice to make these calculations. Maximum and minimum values of the bias drain voltage ( D ). Maximum and minimum values of the bias source voltage ( S ). Maximum and minimum values of the gain O / N if RL = 1 M. Maximum and minimum values of the gain O / N if RL = 7.5 k. Maximum and minimum values of the gain O / N if RL = 1 k. Create a table for prelab and laboratory calculations, and record these values in the table. 8.B. Laboratory Procedure CommonSource Amplifier 8.B.1. Parts Measurement Measure the values of all of your resistors. Record the values in a table and compare them with their 5% limits. 8.B.2. Current Source We will use the circuit below as a current source: 83

4 ECE 25Lab8: SingleStage MOSFET Amplifiers Current 91K 92k MPF12 Figure 81: JFET current source. Right now we do not know how to build a current source, nor do we know how to design with JFETs. Thus, we will just interpret this circuit as a current source and use it for other circuits we are studying. n ECE 351 we will discuss how to build current sources. The circuit of Figure 81 can be interpreted as a the current source in Figure 82 below: Current = 15 Figure 82: Current source. Due to device tolerances, the current of this source can be anywhere between 5 A and 1 ma. The first thing we need to do is wire up the current source and verify that the current through the source is within the tolerance limits. To test this, we will connect the drain of the JFET to ground as shown in Figure 83 below: 91K 92k D G S MPF12 Figure 83: Current source test circuit. Before wiring up the circuit, measure the values of,, and R and fill in Table 81. Make sure that all of the resistor values are correct. 84

5 ECE 25Lab8: SingleStage MOSFET Amplifiers Table 81:Measured values for the current source of Figure 83. alue Measured R 1 91 k R 2 62 k R 12 k CC 15 SS 15 R (oltage across R ) Current R Should be between through R.5 and 1 ma. R Wire the circuit of Figure 83. Measure CC and SS and enter the values in Table 81. Power up the circuit and measure the voltage across R. The current through this source is also the current through R. Calculate the current as the voltage across R divided by R. Enter the values in Table 81. Make sure that the calculated current is between.5 and 1 ma. f not, you have a wiring mistake or a component error. (f your current is not between.5 and 1 ma, do not ask me if it is O.K. to be outside the range. You have a mistake. The circuit has not failed to produce a current between.5 and 1 ma in 25 years of testing!) f your current source appears to be operating correctly, you can proceed to the next section. 8.B.3. Bias erification The first thing we must do is check that the bias is correct. Wire the circuit below and verify that D, S, and D are within the range you calculated in the Prelab. Enter the values in your table. 85

6 ECE 25Lab8: SingleStage MOSFET Amplifiers Rc 7.5k.1u 1u ZN336A MPF12.1u 1u f your bias is correct, you can proceed to the next section. f the bias is not correct, you must have made a wiring error. Do not continue until the bias is correct. 8.B.4. Amplifier Gain Measurement f your bias is correct you can measure the gain characteristics of the amplifier. Wire the circuit below. Note that we have added a 1 to 1 voltage divider. This divider is not part of the amplifier. Since the gain of this amplifier is large, we need a small input so we do not saturate the amplifier. Measure the values of R 4 and R 5 and make sure that they are within 5% of their stated value. 86

7 ECE 25Lab8: SingleStage MOSFET Amplifiers Rc sig R4 1 in R k C6 1U ZN336A out RL.1u 1u MPF12 CE 47u.1u 1u Note that sig and o will be large signals and easy to measure with the oscilloscope. N will be a small signal and hard to measure, but we do not need to measure it because we know the divider ratio of R 4 and R 5, and we are measuring sig. Measure o and sig using the scope, and then calculate the gain as: o in R5 R R 5 4 o sig 1 Measure the gain for RL = 1 M and RL = 7.5 k and compare the values to your calculated values. Record the values in your table. When you measure the gain, make sure that the peaktopeak value of o is less that 1. Record a scope trace of the waveforms you use to measure the gain. 8.B.5. Amplifier Maximum oltage Swing Start with a small value of sig so that the output is an undistorted sine wave. Slowly increase the input until either the top or the bottom of the sine wave clips. Reduce the input slightly so that the output waveform is not clipped. Record the scope waveform and then measure the peaktopeak swing of the output. Measure the maximum swing with no load on the output (remove C 6 and R L and measure the output at the drain of M 1 ) 8.C. Laboratory Procedure SourceFollower Amplifier 8.C.1. Parts Measurement Measure the values of all of your resistors. Record the values in a table and compare them with their 5% limits. o sig 87

8 ECE 25Lab8: SingleStage MOSFET Amplifiers 8.C.2. Bias erification The first thing we must do is check that the bias is correct. Wire the circuit below and verify that D, S, and the current source drain current are within the range you calculated in the Prelab. Enter the values in your table..1u 1u ZN336A MPF12.1u 1u f your bias is correct, you can proceed to the next section. f the bias is not correct, you must have made a wiring error. Do not continue until the bias is correct. 8.C.3. Amplifier Gain Measurement f your bias is correct, you can measure the gain characteristics of the amplifier. Wire the circuit below. Since the gain of this amplifier is close to 1, we do not need to use a voltage divider to reduce the input signal. 88

9 ECE 25Lab8: SingleStage MOSFET Amplifiers.1u 1u ZN336A in Cs 47u MPF12 RL out.1u 1u Measure the gain for RL = 1 M, 7.5 k, and 1 k, and compare the values to your calculated values. Record the values in your table. When you measure the gain, make sure that the peaktopeak value of o is less that 1. Record a scope trace of the waveforms you use to measure the gain. 8.C.4. Amplifier Maximum oltage Swing Start with a small value of in so that the output is an undistorted sine wave. Slowly increase the input until either the top or the bottom of the sine wave clips. Reduce the input slightly so that the output waveform is not clipped. Record the scope waveform and then measure the peaktopeak swing of the output. Measure the maximum swing with no load on the output (remove C S and R L and measure the output at the source of M 1 ). 89