This lab diverges from Horowitz and Hill for the first time this quarter; it is focused on using circuits you have seen previously. The second portion is about iteratively building towards a particular goal (amplifying sound); the layout is similar to the process you would go though when designing circuits for solving real-world problems. There will be overlap with the previous lab; hopefully this will be an opportunity for you to put what you know into practice. There is some new notation in the form of block diagrams, which indicate groups of elements which perform a particular function.

{ ${/download/attachments/205261936/2N3904.png?version=1&modificationDate=1555511845000&api=v2}$

1. Visualizing Current

Consider the circuit shown in Fig. 1, in which three identical LEDs will serve as (rough) indicators of current. Note that the turn-on voltages of these LEDs are approximately 2 V.

1.1.Predict how all three LEDs will compare in brightness.

1.2. Explain how your ranking is consistent with your model for npn transistors.

1.3. Assemble the circuit and check your predicted brightness ranking. Resolve any inconsistencies.

1.4. On the basis of this experiment, what can you infer about the directions of the currents through the base, collector, and emitter? Explain.

Suppose that VCC were reduced from +15 to +3V in this circuit.

1.5. Predict how all three LEDs will compare in brightness. Explain.

Change VCC and test your prediction. Resolve any inconsistencies.

1.6. What behavior regime is the transistor in currently (Active, Saturated, or Cutoff)?

Connect {FIXME $V_{in}$ {FIXME $V_{in}$ to your function generator, set to produce a low frequency (i.e. a few 1 Hz) square wave. Observe the circuit’s output as you adjust the amplitude of the input signal, as well as the voltage {FIXME $V_{CC}$ {FIXME $V_{CC}$ . Are the results consistent with what you expect?

1.7. What must occur for you to observe a crossover between the transistor being active ({ $I_E \approx \beta I_B$ { $I_E \approx \beta I_B$ ) or saturated ({ $V_{CE} \approx 0$ { $V_{CE} \approx 0$ )? How do you know?

Optional Part: LED control techniques

Return {FIXME $V_{CC}$ {FIXME $V_{CC}$ to +15V for this part, and adjust the function generator's frequency.

O.1 At what frequency do you stop seeing individual blinks (on/off cycles) of the lights?

Set the frequency well above that which you found in part O.1, set the function generator to a square wave output, and enable the duty cycle knob (button below power, knob beneath 50k frequency). Look at {FIXME $V_{in}$ {FIXME $V_{in}$ and {FIXME $V_{E}$ {FIXME $V_{E}$ (the emitter voltage) on the oscilloscope as you adjust the duty cycle knob of your function generator.

O.2Briefly describe the effect the duty cycle has on { $V_{in}$ { $V_{in}$ . O.3 What effect does altering the duty cycle have on the brightness of the LEDs? Explain why this happens.

{ ${/download/thumbnails/209518805/Tranistor%20LED%20circuit.png?version=4&modificationDate=1555955800000&api=v2}$ Figure 1: Light Emitting Diodes as current indicators. The brightness of an LED is proportional to the current through it.

{ ${/download/thumbnails/209518805/LED%20Pinout.png?version=3&modificationDate=1555955799000&api=v2}$ A typical LED, along with its circuit symbol. Note that the anode (more positive side) has a leg that is longer than the cathode.

2. Driving low resistance loads

In this section, we will build up to using an BJT emitter-follower circuit to drive the speaker built into your prototyping unit.

To begin, consider the two sub-circuits shown in Fig. 2, which model the function generator as an ideal voltage source with a 50{FIXME $\Omega$ {FIXME $\Omega$ output impedance, and the speaker as a device with a minimum of 8{FIXME $\Omega$ {FIXME $\Omega$ impedance.

2.1 If the output impedance of your function generator is 50 { $\Omega$ { $\Omega$ and the speaker's impedance is roughly 8 { $\Omega$ { $\Omega$ , predict the lowest amplitude that { $V_{spk}$ { $V_{spk}$ could be (e.g. 1/2 { $V_{ideal}$ { $V_{ideal}$ , 1/10 { $V_{ideal}$ { $V_{ideal}$ etc.) when the button is pressed.

Connect your function generator to your speaker via a button. Configure your function generator to produce a sine wave of about 1 kHz, and about 100 mV in amplitude (as low as it goes); you will need to make sure the -20 dB button for this.

2.2 Test your prediction from part 2.1; is the output somewhere between the original { $V_{out,fct}$ { $V_{out,fct}$ and the value you calculated? Note that using too large of an amplitude of signal may result in distortions and displeased looks from classmates. 2.3 Is the speaker's impedance always at least 8{ $\Omega$ { $\Omega$ ? Test a range of frequencies relevant to human hearing (i.e., < 30kHz). 2.4 For what frequency that you tested is the speaker's impedance highest? How do you know experimentally?

{ ${/download/thumbnails/209518805/Model%20Fct%20Gen.png?version=9&modificationDate=1556045410000&api=v2}${ ${/download/thumbnails/209518805/Speaker.png?version=8&modificationDate=1555955800000&api=v2}$ Figure 2: Model of function generator and speaker impedance. Note that our buttons have essentially no resistance when pressed, and (nearly) infinite resistance when not pressed.

{ ${/download/thumbnails/209518805/ButtonConnection.png?version=2&modificationDate=1556045409000&api=v2}$ Button connectivity schematic. To use the button, connect wires across a column or diagonally.

Now, construct an emitter follower circuit, as shown in Fig 3. Note that a 25{FIXME $\Omega$ {FIXME $\Omega$ equivalent resistor is already built on your board.

2.5 Predict what { $V_{out,follower }$ { $V_{out,follower }$ will be if the same input signal { $V_{out,fct}$ { $V_{out,fct}$ from before is used (i.e. the function generator is set to 1kHz, 100mV amplitude). 2.6 Make sure both channels are dc coupled and test your prediction. Briefly describe what you see.

Hint (highlight to read)

The BE junction is never forward-biased here. Recall that the base needs to be at least 0.6 V higher than the emitter for there to be any current in the circuit. Does what you see make sense?

Using the offset knob and button, add a dc offset to your signal until {FIXME $V_{out,follower}$ {FIXME $V_{out,follower}$ has the same ac component as {FIXME $V_{out,fct}$ {FIXME $V_{out,fct}$ .

2.7 What offset was needed to make the emitter follower 'follow'?

{ ${/download/attachments/209518805/FctGenControls.png?version=1&modificationDate=1555955332000&api=v2}$ Function generator controls for duty cycle and offset.

Now construct a biasing network, shown in Fig. 4

2.8 Predict what { $V_{out,bias}$ { $V_{out,bias}$ will be when { $V_{in,bias}$ { $V_{in,bias}$ is a constant dc voltage, and test your prediction.

Attach the output of the biasing network to the input {FIXME $V_{in,follower}$ {FIXME $V_{in,follower}$ of your follower circuit.

2.9 Compare { $V_{out,follower }$ { $V_{out,follower }$ to { $V_{in,bias}$ { $V_{in,bias}$ for a variety of input signals. What function does the biasing network have?

{ ${/download/thumbnails/209518805/25_Ohm_Equivalent2.png?version=1&modificationDate=1555955059000&api=v2}$ Our 25{ $\Omega$ { $\Omega$ resistor, made of eight 220 { $\Omega$ { $\Omega$ resistors in parallel. This is done so that even if we place 5V directly across the resistors, each one only dissipates { $P = (5V)^2 / 220\;\Omega \approx .11 Watts$ { $P = (5V)^2 / 220\;\Omega \approx .11 Watts$ . { ${/download/thumbnails/209518805/Follower%20Chunk.png?version=8&modificationDate=1555955799000&api=v2}$ Figure 3: A basic emitter follower circuit.

{ ${/download/thumbnails/209518805/Biasing%20Network.png?version=7&modificationDate=1556045409000&api=v2}$ Figure 4: A basic biasing network.

Now add a capacitively coupled load to the circuit, shown in Fig. 5.

2.10 Predict what effect will this have on the dc and ac portions of the output { $V_{out, ccl}$ { $V_{out, ccl}$ , then test your predictions. 2.11 What is DC component (mean) of your output now? What is the ac component (amplitude) of your output?

Next, connect the speaker network in place of the 100{FIXME $\Omega \,R_{load}$ {FIXME $\Omega \,R_{load}$ resistor, as illustrated as a block diagram in Fig. 6.

2.12 Predict how the speaker's sound will change compared to what you saw in section 2.2.

2.13 Test your predictions, and comment on any unexpected behavior. What is the ac gain of the circuit now (i.e., the ratio of { $V_{out, follower}$ { $V_{out, follower}$ to { $V_{in,bias}$ { $V_{in,bias}$ )? 2.14 Explain, in your own words, what the function of the transistor circuit was. What problem did it help us solve?

{ ${/download/attachments/209518805/FollowerBlockDiagram.png?version=6&modificationDate=1555955799000&api=v2}$ Figure 6: A block diagram of interconnected circuits. In this representation, sub-circuits are labeled with their function, and only connections between different parts are indicated.

{ ${/download/thumbnails/209518805/CapCoupledLoad.png?version=7&modificationDate=1556045410000&api=v2}$ Figure 5: A capacitively coupled load of 100{ $\Omega$ { $\Omega$ { ${/download/thumbnails/209518805/Speaker%2Ccap%20coupled.png?version=4&modificationDate=1556045410000&api=v2}$ Make sure the speaker network is connected to the emitter of the transistor in the follower network, otherwise it will work very poorly.

3. Amplification

While in theory we could modify the follower we just built into an amplifier, in practice doing so would require very small resistances and very large currents, which is not ideal. Instead, we can build an additional stage to the circuit, amplifying the input signal before the follower.

Keeping what you previously made intact, build a common-emitter amplifier circuit as shown in Fig. 7.

3.1 Predict what the ac gain (ratio of { $V_{in,CEA}$ { $V_{in,CEA}$ to { $V_{out,CEA}$ { $V_{out,CEA}$ ) will be for this circuit, and briefly explain. 3.2 Test your prediction by using the function generator as { $V_{in,CEA}$ { $V_{in,CEA}$ for the input. For what range of input signals does your circuit amplify as expected?

Connect your amplifier to your previous network, as shown in Fig. 8.

{ ${/download/attachments/209518805/Final%20Block%20Diagram.png?version=4&modificationDate=1555955799000&api=v2}$ Figure 8: A block diagram of the final circuits.

3.3 Explain briefly what this circuit does. How does its behavior differ from the follower used in 2.9? (if needs be you can connect your speaker back up to bypass the amplifier for the purpose of making comparisons)

3.4 Predict one modification that would make the speaker quieter overall. Explain your prediction, test the modification, and resolve any discrepancies.

3.5 Predict one modification that would not (typically) affect the speaker's output. Explain your prediction, test the modification, and resolve any discrepancies.

{ ${/download/thumbnails/209518805/CommonEmitterAmplifier.png?version=9&modificationDate=1556045410000&api=v2}$ Figure 7: A common-emitter amplifier circuit. Note that a biasing network is included as part of this circuit

UChicago Instructional Physics Laboratories

Table of Contents

1. Visualizing Current

Consider the circuit shown in Fig. 1, in which three identical LEDs will serve as (rough) indicators of current. Note that the turn-on voltages of these LEDs are approximately 2 V.

Suppose that VCC were reduced from +15 to +3V in this circuit.

Change VCC and test your prediction. Resolve any inconsistencies.

Optional Part: LED control techniques

Return {FIXME $V_{CC}$ {FIXME $V_{CC}$ to +15V for this part, and adjust the function generator's frequency.

2. Driving low resistance loads

To begin, consider the two sub-circuits shown in Fig. 2, which model the function generator as an ideal voltage source with a 50{FIXME $\Omega$ {FIXME $\Omega$ output impedance, and the speaker as a device with a minimum of 8{FIXME $\Omega$ {FIXME $\Omega$ impedance.

Connect your function generator to your speaker via a button. Configure your function generator to produce a sine wave of about 1 kHz, and about 100 mV in amplitude (as low as it goes); you will need to make sure the -20 dB button for this.

Now, construct an emitter follower circuit, as shown in Fig 3. Note that a 25{FIXME $\Omega$ {FIXME $\Omega$ equivalent resistor is already built on your board.

Using the offset knob and button, add a dc offset to your signal until {FIXME $V_{out,follower}$ {FIXME $V_{out,follower}$ has the same ac component as {FIXME $V_{out,fct}$ {FIXME $V_{out,fct}$ .

Now construct a biasing network, shown in Fig. 4

Attach the output of the biasing network to the input {FIXME $V_{in,follower}$ {FIXME $V_{in,follower}$ of your follower circuit.

Now add a capacitively coupled load to the circuit, shown in Fig. 5.

Next, connect the speaker network in place of the 100{FIXME $\Omega \,R_{load}$ {FIXME $\Omega \,R_{load}$ resistor, as illustrated as a block diagram in Fig. 6.

3. Amplification

Keeping what you previously made intact, build a common-emitter amplifier circuit as shown in Fig. 7.

Connect your amplifier to your previous network, as shown in Fig. 8.

UChicago Instructional Physics Laboratories

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Table of Contents

1. Visualizing Current

Consider the circuit shown in Fig. 1, in which three identical LEDs will serve as (rough) indicators of current. Note that the turn-on voltages of these LEDs are approximately 2 V.

Suppose that VCC were reduced from +15 to +3V in this circuit.

Change VCC and test your prediction. Resolve any inconsistencies.

Optional Part: LED control techniques

Return {FIXME $V_{CC}$ {FIXME $V_{CC}$ to +15V for this part, and adjust the function generator's frequency.

2. Driving low resistance loads

To begin, consider the two sub-circuits shown in Fig. 2, which model the function generator as an ideal voltage source with a 50{FIXME $\Omega$ {FIXME $\Omega$ output impedance, and the speaker as a device with a minimum of 8{FIXME $\Omega$ {FIXME $\Omega$ impedance.

Connect your function generator to your speaker via a button. Configure your function generator to produce a sine wave of about 1 kHz, and about 100 mV in amplitude (as low as it goes); you will need to make sure the -20 dB button for this.

Now, construct an emitter follower circuit, as shown in Fig 3. Note that a 25{FIXME $\Omega$ {FIXME $\Omega$ equivalent resistor is already built on your board.

Using the offset knob and button, add a dc offset to your signal until {FIXME $V_{out,follower}$ {FIXME $V_{out,follower}$ has the same ac component as {FIXME $V_{out,fct}$ {FIXME $V_{out,fct}$ .

Now construct a biasing network, shown in Fig. 4

Attach the output of the biasing network to the input {FIXME $V_{in,follower}$ {FIXME $V_{in,follower}$ of your follower circuit.

Now add a capacitively coupled load to the circuit, shown in Fig. 5.

Next, connect the speaker network in place of the 100{FIXME $\Omega \,R_{load}$ {FIXME $\Omega \,R_{load}$ resistor, as illustrated as a block diagram in Fig. 6.

3. Amplification

Keeping what you previously made intact, build a common-emitter amplifier circuit as shown in Fig. 7.

Connect your amplifier to your previous network, as shown in Fig. 8.