Sun Apr 19, 2020 00:34 Lab 4: transistors

Lab 4 Transistor circuits: emitter follower, current source, and amplifier
Sun Apr 19, 2020 00:34
The goal of this lab is to get familiar with transistor operation and the three basic transistor circuits.

Outline: Prelab assignment, Hooking up the power supplies, Selecting the NPN transistor, Emitter Follower, Current source, Common emitter amplifier

Prelab assignment
Read sections 4.1-4.9 of the textbook and note that you did so in your ELog. (Yeah, too easy.)

Hooking up the power supplies

We have only been using the function generator without having much need for the DC power supplies. But, transistors are active components where the output signal has more power than the input signal, so they will need an external DC power supply.
You may have been using the lefthand column of holes on your breadboard for the signal input, but you should now use those columns for the power supplies. You will need to use two power supply channels to get a + and a - power supply. To do this you should use the + end of CH2 as V_CC, i.e., the positive power supply, and the - end of CH1 as V_EE, i.e., the negative power supply. Then define ground as the + end of CH1 and connect that also to the - end of CH2. The picture below shows this.

The connect V_CC to the + rail on your breadboard and V_EE to the - rail. You will also need easy access to ground, so I suggest connecting it to the - rail down the middle of the breadboard. (Just remember to keep track of this being different from V_EE on the other minus rail.) An example is shown in the photo below.

Note that in this lab, and all of the ones using amplifiers, it is increasingly important to keep your circuits "clean". As discussed in the lab 1 instructions, it is best to trim the legs off components so that they sit flush with the surface of the breadboard. This will reduce the contributions from parasitic capacitance and inductance, which can make the amplifiers liable to have a high frequency oscillation noise. You should also connect the scope probes to separate wires rather than the legs of components, otherwise the increasingly complicated circuits can easily be bumped out of connection. An added benefit of cleanliness is that it makes it easier to see your circuit and identify any misconnections that could otherwise take a long time to debug.
You can also reduce the noise in your amplifiers with "by-pass" capacitors. These are capacitors placed between V_CC and ground and then also between V_EE and ground. Use a capacitor value of at least 1 μF and connect it as close to the transistors' (or later op-amps') power connection as possible. These capacitors will act as local energy storage so that any fast switching of current out of the power rails is supplied by the capacitors without having to generate current transients in the wires coming from the power supplies.

Selecting the NPN transistor

Bipolar junction transistors come as either NPN or PNP, with NPN being much more commonly used. We will use the 2N3904 NPN transistors in this week's lab. We also have 2N3906 PNP transistors, but they are different polarity. If they get mixed up, you'll have to look closely at the transistors to read their part number. (I find that zooming with my phone's camera is the easiest way to do that.)
Record in your ELOG the part number (2N3904) that you are using. That may seem like being overly fastidious since we are only going to use one type of transistor. But it is an example of developing good habits in your record keeping. Your records should record what type of transistor (and later what type of opamp) since there can be differences between different implementations.
You should remember which of the pins is the base, the emitter, and the collector from lecture and reading the textbook. This mapping of physical pin number to circuit connection is called "the pinout". We didn't have to worry about that at all for resistors since they work in either direction. And, it was pretty simple to see the polarity marking for diodes. With a three-pin transistor it is only a bit more complicated, but you need to get it right for the circuits to work. Later we will use multi-pin devices where the mapping needs more careful consideration; for those it is worth clipping an image of the pinout into your ELOG for quick reference. If you think it would help to have such a reference for the transistor, go ahead and do that.
Emitter follower

Build the emitter follower circuit below, with R_s=1k and R_E=1k. Note that in this case, V_EE is connected to ground.

This circuit differs slightly from the texbook's circuit on p97 in that it includes a resistor in series with the base. We include that in order to model an output impedance for the signal source. We'll change that resistor to see the effect of the transistor's input impedance. To illustrate that, the schematic below shows how we can think of R_s as part of a "previous circuit stage", where R_s behaves like the Thevenin resistance of the signal source, so it is the output impedance of the signal source. We can then think of the circuit as two stages as illustrated below, with the output impedance of the first stage being R_s and the input impedance of the second (transistor) stage expected to be βR_E as calculated in class.

Power the circuit with V_CC=+3V (and VEE connected to ground). Feed a sinewave with some reasonable amplitude to the input and compare the output voltage to the input voltage. You should see the expected behavior, with the output following the input with a 0.6 V offset, due to the diode drop. As you adjust the amplitude of the input you will see clipping. Add a DC offset to the input signal and watch the clipping stop.
Determine the input impedance of the transistor circuit. You can do this by thinking of the circuit as a voltage divider similar to what is shown below.

Here the point B is the base of the transistor, and R_in corresponds to the input impedance of Stage 2 of the amplifier. You can use the transistor rules to calculate the output voltage in terms of the input voltage to find
V_out(t) = (V_in(t)-0.6)β*R_E/(β*R_E + R_s) = (V_in(t) - 0.6)/(1+R_s/β*R_E)
This is basically just a voltage divider between R_s and R_in where R_in = βR_E plus the extra diode drop.
We expect R_in = βR_E, which is much (100x) larger than R_s. Confirm that the circuit behaves as if the input impedance of the transistor stage of the circuit is much larger than R_s.
Now change R_s to 100k, and you should see that V_out has half the previous magnitude because R_s ≈ βR_E, with β ≈ 100.
Change R_s back to 1k.
If you were to disconnect the power from V_CC, then the transistor no longer can amplify the current, and the base current is the only contribution to the emitter current. Without power, the transistor acts just like a diode between the base and emitter. Try this, and confirm that you still see the 0.6 V diode drop but that the amplitude of V_out no longer gets the current amplification and it acts as if β = 1.
To make sure you understand this, draw the effective circuit diagram that you have with an unpowered transistor.
Finally, reconnect V_CC to the power supply and now connect V_EE to the negative supply which you should set to be -3 V. Make sure that R_s is the original value of 1k. Retest the behavior of the emitter follower circuit and confirm that there is no longer clipping for an input signal that has zero offset voltage, eg
V_in(t) = 0 + 1 sin (ωt)
Modify the DC offset and amplitude of the input signal so it swings close enough to V_EE and confirm that clipping again happens when the input gets within a diode drop of V_EE, even when V_EE is a negative power supply.

Common emitter amplifier

Build a common emitter amplifier with V_CC = +3 V and V_EE = -3 V. Bias the input to put the quiescent point for the emitter at about -2 V. AC couple the input to form a high pass filter with a frequency breakpoint (ie 3dB) of about 1 kHz. Choose R_E and R_C to have a gain of -1.
Test your circuit with a sinewave from the wavegen, and make sure that the quiescent points and gain behave the way you expect.
Now add a gain resistor, R_G and a capacitor, in parallel with R_E to control the gain of the AC signal separately from the DC quiescent points. Choose R_G for a gain of -10 and vary the amplitude of the input signal to observe the gain behavior and the onset of clipping at V_CC and at the V_CE limit.
If you have time, you can try using a triangle wave or square wave input to your amplifier. You may notice that the output shape doesn't match the input shape due to the capacitors filtering the lower frequency components. If you increase the frequency of the input signal these effects will go away.

Current source

Build the current source shown on p99-100 of the textbook. You can measure the current, as we learned to do in lab 1, simply by measuring the voltage across R_E. Using that approach to measure the current, confirm that the current is reasonably constant as you change the load resistance within the compliance range explained in the textbook. You can either do this by swapping in a few different load resistors or by using a trimpots. (To get a varying resistance from the trimpot, connect between the center pin and one of the edge pins not between the two edge pins.)
Note that the best way to maintain a constant base voltage to get a constant load current is by using a zener diode. Well motivated students can look around for a zener diode and try it.