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

Lab 3: Transistor Circuits and JFETsThis Lab is Too Long by 50%3.1 Goals of this LabFollowing our introduction to transistors in Lab 2, this time we will study some commontransistor circuits. Finally, there is a short introduction to one type of transistor of theFET technology family, the JFET. For convenience, the transistor connection conventionsare reproduced below in Fig. 1.Figure 1: Transistor connections.3.2 Emitter FollowerWire up the emitter follower circuit shown below inFig.2. Inputasinewave with amplitudesymmetric about ground. Compare input and output of your follower. Increase the inputamplitude beyond 5 V. Sketch the input and output wave forms.3.2.1Replace the emitter resistor connection to ground with a connection to ,15 V, as shown inFig. 3. Keep the 270 base resistor. (Ignore the dashed boxes for now.) Again look atinput and output wave forms. Sketch the results and note any changes.3.2.2We wish to measure the input and output impedances of our follower. Set up an input sourceand output load circuit as indicated by the dashed boxes in Fig. 3. The capacitor allowsthe DC transistor behavior to remain unchanged, while at higher frequency the capacitorbecomes negligible and the load impedance is approximately given by the 1 k resistor. Tosee this show that the load impedance is R [1 + 1=(!RC) 2 ] 1=2 . Plot this as a function offrequency using the component values for R and C.1

+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

+ 15 VV in0.01 MF2N5485V out1 M 4.7 kFigure 6: Basic JFET source follower.5