2. SINGLE STAGE BIPOLAR JUNCTION TRANSISTOR (BJT) AMPLIFIERS

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1 2. SINGLE STAGE BIPOLAR JUNCTION TRANSISTOR (BJT) AMPLIFIERS I. Objectives and Contents The goal of this experiment is to become familiar with BJT as an amplifier and to evaluate the basic configurations of BJT amplifiers experimentally. The experiment will consist two parts. In the first part, student will analyze a single stage BJT amplifier, however in the second part they are asked to design an amplifier experimentally. II. Equipment and Components: Power Supply Digital Multimeter Oscilloscope Signal Generator Agilent Vee III. Prerequisites: Familiarity to the basic BJT operation is required. I. Theoretical Background a. In this experiment students will investigate a simple BJT amplifier shown in the figure.. Figure BJT Amplifier The npn transistor is connected in a common emitter configuration. The input and output voltage signals are coupled to and from the amplifier with the use of coupling capacitors. The base resistors R and B R bias the transistor to a Q-point. The collector resistor B2 R converts C the output current to an output voltage signal. The amplifier output drives a load with resistance R L. The simple biasing scheme used in this circuit leaves the Q-point sensitive to changes in the value of the transistor b. The small-signal AC equivalent circuit for this amplifier is shown in the figure. Figure 2 AC Equivalent

2 The value of the small-signal input resistance is determined by the DC base current; r = p nvt / I where BQ n is the emission coefficient. The small-signal transconductance is given by g = I / nv. m CQ T b. In the following, some information is presented about design and construct of a single-stage bipolar junction transistor (BJT) amplifier. The elaborated circuit topology is shown in the figure 3. The npn transistor is connected in a common emitter configuration. The input and output voltage signals are coupled to and from the amplifier with the use of coupling capacitors C and C 2. The biasing network consisting of resistors R and R biases the transistor to a DC Q-point. The emitter resistor 2 R = R + R provides bias stability against variations in b. The output of the amplifier is connected E E2 to a load that has a resistance ofr. The total emitter resistance L in series. The second emitter resistor R is bypassed with a capacitor E 2 E gain. However, a small amount of emitter resistance ( is explained later. R is provided by two separate resistors E C in order to increase the voltage R ) is left in the AC circuit, the purpose of which Figure 3 Single stage BJT amplifier The small-signal AC equivalent circuit for this amplifier is shown in the figure. Figure 4 Small-signal AC Equivalent circuit

3 It is assumed that rb = 0, rµ =, r0 =. It is also assumed that all capacitors can be considered to be short circuit at the frequency of operation. The value of the small-signal input resistance is determined by the DC base current r = nv / I. The transistor small-signal transconductance is given by g = I / nv. p T BQ m CQ T It is important to note that the presence of the bypass capacitor C E and the output coupling capacitor C 2 result in the AC load line to be different than the DC load line. In designing the biasing network, it is important to set the Q-point at the center of the AC load line for maximum symmetrical output voltage swing. It is advisable to leave some margin of safety when considering the maximum peak-to-peak undistorted (unclipped) output voltage swing while setting the Q-point. The unbypassed emitter resistor R serves three functions:. Adjust the voltage gain to a desired value: When R = 0, the voltage gain of the amplifier is as high as it can be for the circuit at hand. Setting R to a nonzero value provides a mechanism to decrease the gain to a desired value. 2. Provide stability against variations in r π due to variations in the emission coefficient n: The emission coefficient of a transistor may show a variation from transistor to transistor much like the transistor b. This variation influences the value of r π, which in turn affects the voltage gain and the input resistance of the amplifier. If R is relatively large so that r # = (β + )R +,, the voltage gain and the input resistance are not influenced by changes in the value of n. 3. Provide linear (undistorted) operation for larger input signals: When R = 0, the input voltage is limited to values that are much smaller than V for linear (undistorted) operation. This is so because the small-signal approximation exp 0 2 = + v 3 67 /V : fail unless v 67 V :. However, 4 if R is relatively large so that r # = β + R +,, the base current is mainly determined by the linear resistance ( b + ) R E, rather than the nonlinear i-v curve approximated by the resistance r π. II. PRELIMINARY WORK. Each of the students is tested before starting experiment by research assistant. This test may be written or oral examination. Theoretical knowledge of the experiment that should be gotten from experimental sheet and other sources (lecture notes, books etc.). The theoretical information about experiment is not limited to study only experimental sheet, students have to research other sources to get enough knowledge. 2. Students should know the purpose of the experiment. They should know how the experiment can be done and which measuring elements can be used. They should also get measuring elements catalog information. 3. Read the datasheet of BC238 transistor series. What is the typical base to collector amplification factor of BC238 BJT? What does the gain-bandwidth product mean? 4. Please answer the following questions before attending the Lab. What is an amplifier? Which parameters affect the performance of an amplifier? What are the main features of single-transistor amplifier stages? T What are the important frequency values of gain-frequency curve of an amplifier? How are they obtained?

4 5. In the laboratory you will construct the amplifier circuit using the following values (for the first experiment Figure. and Figure.2): Table. V CC R B R B2 R C R L C 5 V 470 kω.8 kω 470 Ω 00 kω 0 µf The transistor that you will use is BC238B. This transistor has 200 < b < 320. In the preliminary work section, you are asked to base your calculations on three different values of b, namely b = 250, b 2 = 200, andb 3 = 320. Other transistor parameters are VCE ( SAT ) = 0.2V and VBE ( ON ) = 0.6V. Assume that the emission coefficient n =, even though BC238B transistors usually exhibit a higher value. For each b value (250, 200, 320): 6. Analyze the DC circuit to determine the Q-point. Find I BQ, I CQ, and V CEQ. 7. Draw the load line and the transistor i C v CE characteristics, and indicate the Q-point. (Separate graph for each β value.) 8. Calculate the peak-to-peak maximum undistorted voltage swing at the output. 9. Draw the AC equivalent model assuming that the capacitors are short circuit at the operating frequency. Calculate r, g, and the the voltage gain A p m v = v out /v in. 0. Simulate the amplifier circuit using LTSPICE. First do a Bias Point analysis to determine DC voltages and currents. Next, do a Time Domain analysis using an input sine wave of 2 mv peak-to-peak at a frequency of khz, and determine the voltage gain. Repeat with input amplitudes of 40 mv, 40 mv, and 500 mv; note the changes on the output waveform. Comment on your results. EXPERIMENT-I (Figure. and Figure.2) In this experiment you are going to use the silicon npn transistor BC238B. Please check the datasheet of BC238 before attending the laboratory. For this transistor, V <+(=>) = 0.6V and V B+(CD:) = 0.2V. Before constructing the circuit, verify the values of the resistors. Make sure that all resistors are within 2%-%5 of their marked values. This will assure that your current measurements are accurate. Construct the amplifier circuit using the values indicated in the preliminary work section.. Before connecting the signal generator, measure I BQ, I CQ, and V CEQ, and compare these with your calculations. Draw the load line on a graph and indicate the Q-point. 2. Using your measurements from the previous part, determine the β of the transistor that you are using. 3. Now you have measured the particular β (for n=.4) values for the specific transistor that you are using. Calculate the voltage gain of the amplifier using these values. 4. Set the input voltage signal to a sinusoid with khz frequency and 00 mv peak-to-peak amplitude. Measure the voltage gain of the amplifier and compare with your calculations from the previous part. In that step, you should also use Agilent Vee pro. In order to apply a signal

5 5. with different frequency (For instance you may apply between 00 Hz- 0 Khz). Observe the input and output voltage waveforms on the oscilloscope. 6. Gradually increase the input signal amplitude and determine the onset of distortion at the output. 7. Further increase the input signal amplitude and determine the onset of clipping. Measure the peak-to-peak maximum undistorted output voltage swing. (Here undistorted really means unclipped. ) Comment on how this value is related to the location of the Q-point on the load line. 8. Print the output waveform for three different values of peak-to-peak input amplitude: 40 mv, 40 mv, and 500 mv. On each plot, indicate any distortion or clipping that you may see.

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