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Results:
Based on measurements, complete the table below:
Discussion:
Discuss the application of common collector transistor circuit.
The common collector configuration of the BGJ amplifier is mostly commonly employed as buffer or emitter follower. The voltage gain of the common collector amplifier is nearly unity and provides large current amplification. It is used in applications where current sourcing is required without modifying the voltage. The input resistance is quite high so it draws negligible current from source. Moreover, the load circuit will not interfere with the source driving it because of the high input impedance of the voltage follower. The output resistance is very low so that loads requiring large current can be fed from emitter terminal.
The common collector can be used to run large loads such as motors from sources such as microcontrollers that cannot source currents. The current sourcing will be provided by the common collector configuration.
Calculate the DC emitter current IE and internal emitter resistance for the circuit.
In order to find the DC emitter current IE, we need to perform DC analysis of the common collector BJT amplifier. The capacitors will be assumed open circuits for DC analysis.
The resistances R1 and R2 comprise voltage divider circuit for DC biasing on input base terminal of common collector BJT. Therefore, the input voltage will be:
Vin=10k10k+47k×Vcc=10k10k+47k×15=2.6316V
The voltage drop across base-emitter junction is 0.7V. So, we have DC output voltage at emitter terminal as:
VE=Vin-VBE=2.6316-0.7=1.9316V
The DC emitter current IE is calculated as follows:
IE=VERE=1.93161k=1.9316mA
The internal emitter resistance is found from the voltage drop across it. The total voltage drop from base to emitter (VBE) is 0.7V. Some part of it is dropped across internal base resistance and the rest is dropped across internal emitter resistance. Since, internal emitter resistance is quite small so it is assumed that one third of VBE drops across it. The current flowing through emitter is already calculated above. Hence, internal emitter resistance will be:
re=13×0.71.9316mA=121Ω
Using the values obtained in the experiment; draw the small-signal ac equivalent circuit for the amplifier.
Using the value of internal emitter resistance, we draw the small-signal AC equivalent circuit for the common collector amplifier. The equivalent circuit is obtained by replacing DC sources and capacitors by short circuits. The resulting circuit is shown below:
Using the equivalent circuit, calculate the theoretical values for AV, rin(stage), and ro(stage).
We have the small signal AC equivalent circuit as shown above. It can be used to find out the open loop voltage gain, input and output resistance. The voltage gain of the common emitter amplifier can be found using the following formula:
Av=REre+RE=1k1k+120=0.893
Ideally the open loop voltage gain of the emitter follower should be unity. The gain of 0.893 is quite close to the practical value of 0.95. The value is smaller than unity because the emitter resistance used for DC biasing is not large enough as compared to internal emitter resistance and input voltage is also small. The value of internal resistance of small signal AC equivalent model is found as follows:
rinstage=R1//R2//β(re+RE//RL)
Since, load resistance is not provided, so we ignore it in the calculation of the internal resistance. The formula becomes:
rinstage=R1//R2//β(re+RE)
rinstage=10k//47k//197(120+1k)
rinstage=8.2456k//220.640k
rinstage=7.95kΩ
Output resistance can be calculated from the following formula:
rostage=RE//(re+R1//R2//rsβ)
rostage=1k//(120+10k//47k//rs197)
Internal resistance is not provided, so it will be assumed to be very small.
rostage=1k//(120+0)
rostage=107.143Ω
Large internal resistance and small output resistance is the typical characteristic of emitter follower.
