Lab 3: Transistor Circuits and JFETs This Lab is Too Long by 50 3.1 ...

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+15 V2702N3904Vin3.3kVoutFigure 2: Basic emitter follower.+15 VVin10 k2N39043.3kVout+3.3 MF1 k-15 VFigure 3: Symmetric follower with input (source) and output (load) stages shown in thedashed-line boxes.2
To measure Z out ,we consider the Thevenin equivalent circuit with the load, consisting ofthe 1 k load in parallel with the 3:3 k emitter resistor, in series with Z out , hence formingavoltage divider. Operating at a frequency well above ! 3db of the load, measure the outputamplitude with and without the 1 k load to determine Z out .To measure Z in , remove the load. The entire follower now represents an impedance Z inin series with the 10 k source resistor. Hence, by measuring the signal amplitude on eachside of the 10 k resistor, we can determine Z in .Thismay be dicult to measure.An important point of the emitter follower is that it represents a large input impedanceand a small output impedance. The input impedance is about Z in Z load , whereas Z out Z source =, where Z load is the load to be driven by the emitter-follower including any emitterresistor (3:3 k here), and Z source is the source impedance of the signal being delivered tothe emitter-follower. Do these expectations agree with your measurements ?3.3 Current SourceThe arrangement shown in Fig. 4 (left) can serve asanapproximate current source. Usethe 10 k variable resistor on your prototype board as the load to be driven by the currentsource. Again, measure the collector current delivered to the load by the voltage drop acrossthe 1 k resistor. Starting with the highest value, vary (slowly!) the load resistance whilemeasuring the current. How good is this current source? At whatvalue of R does thedelivered current deviate by more than 20% from the initial value?+15 V+15 V1k6.8 k(load) R0.25 MF56 kVoutVin2N3904+5 V4702N39045.6k820680Figure 4: A simple current source (left), and a common emitter amplier (right).3.4 Common-emitter AmplierSet up the common-emitter amplier shown in Fig. 4 (right). Input a small-amplitude sinewave. Measure the voltage gain of the amplier V out =V in over the frequency range 50 Hz to10 kHz. Make a rough plot of gain versus log 10 of frequency. What is the measured ! 3dbpoint? Does this agree with your expectation?3
Page 1: Lab 3: Transistor Circuits and JFET

Lab 3: Transistor Circuits and JFETs This Lab is Too Long by 50 3.1 ...

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