Lab 3
...nal VBE is greater than 0.7 volt in the vicinity of 0.75 to 0.78 volt. For DC currents, the capacitors are open circuits and the resisters R4 and R5 are in series with an equivalent value of RE. The resistors: R1, R2, R3 and RE form a four resistor-biasing network. The capacitor C1 couples the signal source to the base of the transistor, and C2 couples the amplified signal at the collector to the load RL. The capacitor CE is called a bypass capacitor; it provides a low-impedance path for the ac emitter current to ground. To get a grasp on how the common emitter circuit works, we will analyze the Thevenin equivalent circuit known as a four-resistor biased circuit, Figure 1. The Thevenin resistance RB of the four-resistor biased circuit is found by the parallel combination of R1 and R2 (2). The Thevenin voltage is can be calculated using (3). Figure 1: Four resistor biasing circuit (2) (3) To find voltages across the different resistors under the assumption that b is large enough to ignore IB; we will use Ohm’s Law (4). (4) The following equations were used to obtain resistor voltages using the concept of Ohm’s Law. (5) (6) (7) (8) Next, we recalculated the previous DC quantities using a more realistic assumption of b= 75. The base current IB was first calculated using Equation 8 and the collector current IC was calculated using Equation 9. IB and IC were then substituted in Equation 1 thru Equation 7 and were compared to the DC quantities calculated without b. (9) The collector current is an amplified version of the base current. (10) Last we calculated the gain of the circuit Figure 1. First we assumed that b is large enough that (b+1) RE >> Rp and that (b+1) b using (11). Then we calculated the gain again assuming that b=75 using Equation 11 and are compared Table 1. (11) (12) Theoreticalwithout b Theoreticalwith b -8.91 -8.00 Table 1: Calculated gain The input impedance for a common emitter can be found by looking into the base terminal of the transistor. (13) The input impedance seen by the source is the parallel combination of RB and Zit. (14) The current gain, Ai, can be found by using 15. The power gain, G, of the common emitter circuit can be found by the product of the current gain and voltage gain 16. (15) (16) Procedure The following equipment was used in this laboratory: · Transistor: 2-npn 2N2222A/ZTX · Resistors: 2-20k W, 10k W, 8.2k W, 5.6k W, 2-1k W, 800 W, 100 W · Capacitors: 3-3.3mF · 1 breadboard · 1 Function generator · 1 DC power supply · Agilant Oscilloscope · Two sets of probes · Three sets of cables The first part of the experiment consisted of constructing a common emitter circuit, as seen in Figure 2. Figure 2: Common Emitter first stage The voltage gain, Avce-m, input and output impedances, Rince-m , Routce-m, Current gain, Aice-m, and Power gain, Apce-m, were measured. The second part of this experiment consisted of constructing the common collector biasing circuit as seen in Figure 3. The dc voltages were measured. Figure 3: Common Collector biasing circuit The third part of the experiment required combination of both the emitter follower and common emitter without RL connected Figure 3. The input and output impedances current gain, voltage gain, and the power gain were all measured from the common collector. Then, the voltage gain was measured on the common emitter. Next, the total power gain, total voltage gain was measured for the two stages based on the voltage delivered to R9, Figure 4. Figure 4: Two-stage amp RL not connected Last, RL was connected to the two-stage amplifier. The input and output impedances current gain, voltage gain, and the power gain were all measured from the common collector. Then, the voltage gain was measured on the common emitter. Next, the total power gain, total voltage gain was measured for the two stages based on the voltage delivered to R9, Figure 5. Figure 5: Two-stage amp RL connected Data and Analysis In the first part of the lab, in order for us to find the values for the input and output, we used a method from the pre-lab. We placed a resister, Rtest, with a value we assumed was the impedance of the circuit. We measured vi and vo from measure the voltage from the positive side Rtest to ground and then from the negative side of Rtest to ground. This, in essence is a voltage divider. We compared the theoretical values calculated earlier with those measured assuming b= 75. Calculated Measured % Error Avce -7.26 -7.02 3.30 Rince 2715.83 3348 23.28 Routce 1000 988 1.2 Aice -4.93 -3.40 30.94 Apce 35.83 23.90 3...