Lab #5

Simple Transistor Circuits

In this lab you will build some elementary transistor circuits. While in the real world, designing circuits at the transistor level is often unnecessary due to the availability of more sophisticated and convenient integrated circuits, transistors are the essential ingredient in all such devices. Design using discrete (i.e. individual) transistors is still an important technique when working with circuits that require high power or high voltage or operate at high frequencies. The transistor used in this lab is a standard bipolar NPN device.

1) Type determination: Verify that the transistor is in fact an NPN type using an ohmmeter,regarding it as two back-to-back diodes. Explain how you do this and report measured resistances.

2) Measuring h_fe (or ): Recall that for a transistor I_c = h_feI_b. Use the basic circuit below to measure I_c for various I_b. Change R to obtain different I_b, using the values R = (5MΩ, 1MΩ,500kΩ, 100kΩ, 50kΩ, 20kΩ and 10kΩ). Describe how you measure both I_c and I_b. From a linear/linear plot of I_c vs. I_b estimate h_fe () for your transistor. Also make a plot of log(I_c) vs. log(I_b) (measure both currents in mA for consistency), and determine a value of  from this graph as well. The 500Ω resistor between the power supply and the collector is present to limit the collector current.

3) A transistor switch and transistor saturation: Verify that with I_b=0 in the circuit below the light bulb is off and that it turns on when the switch is closed. The 50Ω resistor again serves to limit the current to the bulb, otherwise it might burn out. Measure I_b and I_c. Also measure the voltage drop between the collector and emitter of your transistor, V_ce. Replace the 1kΩ resistor with 10kΩ, 100kΩ, and 220Ωresistors and repeat the measurements of I_b, I_c, and V_ce. Make a plot of V_ce as a function of I_c. Where in this plot does transistor saturation occur? From your measurements, estimate the resistance of the light bulb when it's on and when it's off (not glowing).

4) Emitter follower: Build the emitter follower shown, using V_cc=+5V and the function generator (2V_pp, 5kHz) as V_in. Be sure that the DC offset of the function generator is off. Use the scope to observe the waveform with the scope input DC coupled. Observe V_out: include quantitative sketches of the input and output. Then vary the amplitude of V_in and report your observations. For what input voltages does “clipping” of the waveform occur?

Try again with V_cc=+5V, but let V_ee=-15V, instead of connecting the emitter resistor to ground. Be careful about where you ground the low side of the scope and function generator leads for this case; don't connect them to the emitter resistor. Vary the amplitude and DC offset of the input and observe the output. Quantitatively sketch the input and output for a situation in which there is no clipping and again when there is clipping apparent in the output. Why does clipping occur? Explain the differences in behavior of the circuit for the emitter resistor connected to ground or V_ee=-15V.

5) Common emitter amplifier: Build the simple amplifier shown below. Let R₁=100kΩ,R₂=10kΩ, R_c=5kΩ. Use +12V or +15V for V_cc. Choose R_e to give a gain of approximately 10 for the amplifier. Choose C_in to give a 3 dB point for the input of 100 Hz (Make the input act like a high pass filter. The effective resistance for ac signals applied to the input is the parallel combination of R₁,R₂, and R_e - which resistance dominates this combination?). Use C_out=.01F. Estimate what the quiescent current is (I_c or I_e without V_in applied), based on the biasing network of R₁ and R₂ and the value you chose for R_e. Verify your estimate by measuring V_e to find I_e.

Apply a small AC signal (V_in = 0.1V_pp @5kHz) to the amplifier. Use a voltage divider if the output of the function generator can't be made small enough. Make quantitative sketches of V_in and V_out. Is there a phase shift?

Vary the frequency over the range 1Hz to 1 MHz (in 1-3-10-... steps) and plot the frequency response (|v_out/v_in| vs. log(f)) of your amplifier. Does it behave as designed? If you observe deviations, try to explain them. Vary the amplitude of the input. Do you observe clipping? If so, for what input amplitudes? For the positive or negative portion of the waveform? Why does it occur?