Lab 3: Transistor Circuits and JFETs This Lab is Too Long by 50 3.1 ...
Lab 3: Transistor Circuits and JFETs This Lab is Too Long by 50 3.1 ...
Lab 3: Transistor Circuits and JFETs This Lab is Too Long by 50 3.1 ...
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To measure Z out ,we consider the Thevenin equivalent circuit with the load, cons<strong>is</strong>ting ofthe 1 k load in parallel with the 3:3 k emitter res<strong>is</strong>tor, in series with Z out , hence formingavoltage divider. Operating at a frequency well above ! 3db of the load, measure the outputamplitude with <strong>and</strong> 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 res<strong>is</strong>tor. Hence, <strong>by</strong> measuring the signal amplitude on eachside of the 10 k res<strong>is</strong>tor, we can determine Z in .<strong>Th<strong>is</strong></strong>may be dicult to measure.An important point of the emitter follower <strong>is</strong> that it represents a large input impedance<strong>and</strong> a small output impedance. The input impedance <strong>is</strong> about Z in Z load , whereas Z out Z source =, where Z load <strong>is</strong> the load to be driven <strong>by</strong> the emitter-follower including any emitterres<strong>is</strong>tor (3:3 k here), <strong>and</strong> Z source <strong>is</strong> 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 res<strong>is</strong>tor on your prototype board as the load to be driven <strong>by</strong> the currentsource. Again, measure the collector current delivered to the load <strong>by</strong> the voltage drop acrossthe 1 k res<strong>is</strong>tor. Starting with the highest value, vary (slowly!) the load res<strong>is</strong>tance whilemeasuring the current. How good <strong>is</strong> th<strong>is</strong> current source? At whatvalue of R does thedelivered current deviate <strong>by</strong> 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), <strong>and</strong> 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 <strong>50</strong> Hz to10 kHz. Make a rough plot of gain versus log 10 of frequency. What <strong>is</strong> the measured ! 3dbpoint? Does th<strong>is</strong> agree with your expectation?3
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