Prelab 6: Biasing Circuitry

Transcription

1 Prelab 6: Biasing Circuitry Name: Lab Section: R 1 R 2 V OUT Figure 1: Resistive divider voltage source 1. Consider the resistor network shown in Figure 1. Let = 10 V, R 1 = 9.35 kω, and R 2 = 650 Ω. We can apply Thévenin s equivalent circuit to this resistive divider to model it as an independent voltage source with an output resistance. Find the open circuit voltage and the output resistance for this resistive divider. 2. Now, consider a BJT voltage source such as the one shown in Figure 2. Pick the appropriate value for R C so as to achieve an output voltage of 650 mv. Let = 10 V, I S = fa, and V T = 26 mv. Ignore the Early effect for this calculation. 1

2 2 R C I C V OUT Figure 2: BJT voltage source 3. Calculate the output impedance of this BJT voltage source and determine the power dissipated by the circuit. Hint: Recall the definition of power, P = IV. 4. If we resized the resistors in the resistive divider (shown in Figure 1) to achieve the same output impedance as the BJT voltage source, what should the values of the two resistors be so that the output voltage stays the same? How much power will the circuit now consume? R I OUT V OUT R L Figure 3: Resistor current source 5. Consider the circuit shown in Figure 3. Let = 10 V and R = 10 kω. Roughly sketch I OUT vs. V OUT (space is provided on next page). Note: The specific value for R L is not required for this analysis.

3 IOUT (ma) V OUT (V) 6. For the current source in Figure 3, find the short-circuit output current as well as the output impedance. Note: R L represents a hypothetical load to the circuit; it can be replaced with an open-circuit for the appropriate analysis. 7. For a given load, is it possible to increase the output impedance of the current source without decreasing the output current and without changing? Explain. c University of California, Berkeley 2008 Reproduced with Permission Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc. This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor and Agilent are providing this experiment solely as an informational facility and without review.

4 NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDI- RECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION WITH THE USE OF THIS EXPERIMENT. 4

5 Experiment 6: Biasing Circuitry 1 Objective Setting up a biasing network for amplifiers often involves using transistors to build voltage and current sources to deliver the right amount of biasing to the main amplifier components. In this lab, we explore how to build these sources and fine tune their output values to achieve the optimal biasing environment. 2 Materials The items listed in table 1 will be needed. Note: Be sure to answer the questions on the report as you proceed through this lab. The report questions are labeled according to the sections in the experiment. CAUTION: FOR THIS EXPERIMENT, THE TRANSISTORS CAN BECOME EXTREMELY HOT!!! Component Quantity 100 Ω resistor 1 1 kω resistor kω resistor 1 10 kω potentiometer 1 2N4401 NPN BJT 1 2N4403 PNP BJT 2 Table 1: Components used in this lab 3 Procedure 3.1 Emitter Degeneration in Current Sources In this section, we will examine the effect of emitter degeneration on a current source. 1. Build the circuit shown in Figure 1, setting = 5 V and V BIAS = 4.35 V. As you adjust V BIAS, be sure to check that V BE stays less than or equal to 0.65 V at all times; otherwise, your transistor may BURN UP! (Warning: It is very easy to make a mistake and burn out a transistor. Therefore, it is highly recommended that you check your values for V BIAS and with the multimeter or oscilloscope before connecting them to the transistor.) 2. Now measure the short circuit output current. 3. In terms of the small-signal characteristic(s) (e.g. g m, r o, β), what is the output resistance of this current source? 4. Using ICS, sweep V OUT from 0 V to 5 V and plot I OUT vs. V OUT. What is the output impedance at V OUT = 2.5 V? What happens to the output impedance as V OUT approaches 5 V? Explain your observation. 1

6 3 PROCEDURE 2 V BIAS I OUT V OUT Figure 1: Transistor current source 100 Ω V BIAS I OUT V OUT Figure 2: Current source with emitter degeneration 5. Now set up the circuit as shown in Figure 2 by adding a resistor to the emitter. Carefully adjust the bias voltage, V BIAS, to maintain the same short circuit output current. 6. Using ICS, measure the output impedance for this modified circuit. Explain how the additional resistor affects the output impedance. Note: Recall the output impedance is measured from the inverse slope of the I OUT vs. V OUT curve, not the absolute values of I OUT or V OUT. 3.2 Current Mirror In this part of the lab, we create a current mirror. The resultant copy current can be used to bias other circuit elements and semiconductor devices. 1. Before building the circuit for this part of the lab, be sure to setup your potentiometer so that the two leftmost terminals have the MAXIMUM possible resistance across them (which should be about 10 kω). Then, use these terminals for your initial R C. For assistance, please refer to Figure 3, which illustrates a typical potentiometer. Please be absolutely sure that you use these terminals as the initial resistance for your circuit; otherwise your transistor may BURN UP!

7 3 PROCEDURE 3 Figure 3: A typical potentiometer Q REF I REF V OUT R C Figure 4: Voltage source using a transistor 2. Now build the circuit shown in Figure 4, setting = 5 V. 3. Adjust the potentiometer until I REF is approximately 2.2 ma. What is the value of R C at this point? 4. Perform load-line analysis by sketching the I REF vs. V OUT curves for both the transistor and R C. On your sketch, indicate the fixed point solution for I REF. How should we adjust the potentiometer to increase I REF (i.e. do we increase or decrease the value of R C )? Q REF Q COPY I REF I COPY R C 1 kω Figure 5: PNP current mirror 5. Now add an PNP BJT and a 1 kω resistor to your circuit so that it looks like Figure Measure the value of I COPY (the copy version of I C ).

8 3 PROCEDURE 4 7. Do I COPY and I REF match? If not, what are some possible reasons for this discrepancy? Hint: Begin by considering the base currents of the BJTs. 3.3 Biasing a Common Emitter Amplifier Using a Current Mirror A current mirror is simply a series of current sources biased by a voltage source (see Figure 6). This circuit is useful for replicating a biasing current that is used in many places throughout your overall circuit design. As shown in Figure 7, we can use a current mirror to bias a common emitter amplifier.... R C Figure 6: General PNP current mirror Q2 Q3 I C2 I C3 R C v in V IN R E Q1 V out Figure 7: Biasing a CE amplifier with a current mirror 1. Before building the circuit for this part of the lab, be sure to setup your potentiometer so that the two leftmost terminals have the MAXIMUM possible resistance across them (which should be about 10 kω). Then, use these terminals for your initial R C. Now, build the circuit shown in Figure 7, setting = 12 V, R C = 10 kω potentiometer, and R E = 100 Ω. 2. Use the DC offset from the function generator to set V IN to 760 mv and also apply a small signal to the input. Approximately, what is the gain of the amplifier in Figure 7? Note: Recall that the actual output from the function generator is doubled the amount on the display. 3. Use the function generator to apply a 20 mv pp, 1 khz sine wave at the input and plot the output on the oscilloscope. Because of the huge gain, the output waveform may be clipped. Attach a 100 kω load

9 3 PROCEDURE 5 resistor at the output (which should make the output no longer clipped) and measure the peak-to-peak voltage across the resistor. Using the value of this voltage and the gain previously measured, calculate the output impedance of the amplifier. 4. Qualitatively, how do the measured output impedance and gain compare against a common emitter biased by a resistor? 5. Now, let us examine the effects of variations in biasing point: (a) Adjust the potentiometer so that R C is slightly smaller. Observe how this affects the output waveform. Please be careful to NOT burn up your BJTs while adjusting the potentiometer. (b) Qualitatively, how does changing R C affect I C2 and I C3 (i.e. does decreasing the value of R C increase or decrease I C2 and I C3 )? Based on your response, explain how changing R C changes the output waveform? c University of California, Berkeley 2008 Reproduced with Permission Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc. This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor and Agilent are providing this experiment solely as an informational facility and without review. NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDI- RECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION WITH THE USE OF THIS EXPERIMENT.

10 Report 6: Biasing Circuitry Name: Lab Section: 1 Lab Questions Measure the short circuit output current. : I OUT = In terms of the small-signal characteristic(s) (e.g. g m, r o, β), what is the output resistance of this current source? Theoretical R out = What is the output impedance at V OUT = 2.5 V? What happens to the output impedance as V OUT approaches 5 V? Explain your observation. At V OUT = 2.5 V, measured R out = Measure the output impedance for this modified circuit. Explain how the additional resistor affects the output impedance. R OUT = Adjust the potentiometer until I REF is approximately 2.2 ma. What is the value of R C at this point? 1

11 1 LAB QUESTIONS 2 R C = Perform load-line analysis by sketching the I REF vs. V OUT curves for both the transistor and R C. On your sketch, indicate the fixed point solution for I REF. How should we adjust the potentiometer to increase I REF (i.e. do we increase or decrease the value of R C )? Measure the value of I COPY I COPY = Do I COPY and I C match? If not, what are some possible reasons for this discrepancy? Hint: Begin by considering the base currents of the BJTs Properties of the CE amp with current mirror: A v = R out = Qualitatively, how do the measured output impedance and gain compare against a common emitter biased by a resistor? Qualitatively, how does changing R C affect I C2 and I C3 (i.e. does decreasing the value of R C increase or decrease I C2 and I C3 )? Based on your response, explain how changing R C changes the output waveform?

12 1 LAB QUESTIONS 3 c University of California, Berkeley 2008 Reproduced with Permission Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc. This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor and Agilent are providing this experiment solely as an informational facility and without review. NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDI- RECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION WITH THE USE OF THIS EXPERIMENT.