Date……………………………. Roll No. ……………………………….
EXPERIMENT 5
Darlington and Cascode Amplifier Circuits
Objective
1. To calculate and measure DC and AC voltages in Darlington and cascade connection circuits. Equipment
1. 2. 3. 4.
Oscilloscope DMM Function generator DC supply
Components
1. Resistors [100Ω, (1); 51Ω51Ω -1W, (1); 1kΩ, (1); 1.8kΩ, (1); 4.7kΩ, (1); 5.6kΩ, (1); 6.8kΩ, (1); 50kΩ pot, (1); 100kΩ, (1)] 2. Capacitors [0.001µF, (1); 10µF, (4)] 3. Transistors [2N3904,(or equivalent general purpose npn), (2); TIP120 (npn Darlington), (1)] Discussion
Darlington Circuit: A Circuit: A Darlington connection co nnection (as shown in Fig. 23.1) provides a pair of BJT transistors in a single IC package with effective beta ( equals to the product of the individual transistor betas: 23.1 The Darlington emitter-follower has higher input impedance than that of a n emitter-follower. The Darlington emitter-follower input impedance is:
23.2
The output impedance of the Darlington emitter-follower is
23.3 The voltage gain of a Darlington emitter-follower circuit is: 23.4 Cascode Circuit: A Cascode circuit , as shown in Fig. 23.2, provides a commonemitter amplifier using Q 1 directly connected to a common-base amplifier using Q2. The voltage gain of stage Q 1 is approximately 1, with the voltage V o1 being opposite in polarity to that applied as V i. 23.5 The Voltage gain of Q 2 is non-inverted and of magnitude
23.6
Resulting in an overall gain:
23.7
Procedure: The students are expected to condu ct the foll owin g in str ucti ons fi rst i n PROTE US and then experi mental ly in the lab du r in g the same lab sessi on.
Part 1. Darlington Emitter-Follower Circuit
a. For the circuit of Fig. 23.1 calculate the DC bias voltages and currents. +Vcc ( 10V ) RB(variable) 2MΩ Vi
C B
TIP 120
C1 E
0.001 µF
C2 RE 390Ω Figure 23-1
1/2W
10 µF
(calculated) = __________ (calculated) = __________ Calculate the theoretical values of voltage gain, input and output impedance.
(calculated) = __________ (calculated) = __________ (calculated) = __________ b. Construct the Darlington circuit of Fig. 23.1. Adjust the 2M Ω potentiometer ( to provide an emitter voltage, = 5V. Using a DMM, measure and record the DC bias values:
(measured) = __________ (measured) = __________ Calculate the base and emitter DC currents:
(calculated) = __________ (calculated) = __________ Calculate the value of transistor beta at this Q-point:
(calculated) = __________
c. Apply an input signal = 1V, peak at 10 kHz. Using the oscilloscope observe and record the output voltage to ass ure that signal is not clipped or distorted. (Reduce the input signal amplitude if necessary.)
(measured) = __________ (measured) = __________ Calculate and record the AC voltage gain:
__________
Part 2. Darlington input and Output Impedance
a. Calculate the input impedance:
(calculated) = __________ Calculate the circuit output impedance:
(calculated) = __________ b. Connect a measurement resistor = 100 kΩ, in series with . Measure and record input voltage .
(measured) = __________ Calculate the circuit input impedance using
(calculated) = __________ Remove measurement resistor, . c. Measure the output voltage, with no load connected.
(measured) = _________ Connect load resistor, 100Ω. Measure and record res ulting output voltage.
(measured) = _________ Calculate the output impedance using:
(calculated) = _________ Compare the calculated and measured values of and .
Part 3. Cascode Amplifier
a. Calculate DC bias voltages and currents in the cascode amplifier of Fig. 23.2 (assuming base currents are much less than the voltage divider current).
+Vcc ( 10V ) RB1
RC
6.8 kΩ
1.8 kΩ
C3
C2
Vo 10 µF
10 µF
RB2
00
5.6 kΩ
C1
Figure 23-2
Vi
00
10 µF RB3
RE
4.7 kΩ
1 kΩ
CE 10 µF
(calculated) = __________ (calculated) = __________ (calculated) = __________ (calculated) = __________ (calculated) = __________ (calculated) = __________
Calculate the DC bias emitter currents:
(calculated) = __________ (calculated) = __________ Calculate the transistor dynamic resistances:
(calculated) = __________ (calculated) = __________ b. Connect the cascode circuit of Fig. 23.2. Measure and record DC bias voltages.
(measured) = __________ (measured) = __________ (measured) = __________ (measured) = __________ (measured) = __________ (measured) = __________ Calculate the values of emitter currents:
= __________ = __________
Calculate the transistor dynamic resistances:
= __________ = __________
c. Using Eqs. 23.5 and 23.6 calculate the AC voltage gain of each transistor stage:
(calculated) = __________ (calculated) = __________ d. Apply an input signal = 10mV, peak at 10 kHz. Using the oscilloscope observe the output waveform to assure that signal is not clipped or distorted. (Reduce the input signal amplitude if necessary.) Using the DMM measure, record the AC signals.
(measured) = __________ (measured) = __________ (measured) = __________ Calculate and record the AC voltage gain:
__________
__________ __________
Compare the measured voltage gains with those calculated in steps 3(c) and 3(d).
e. Using the oscilloscope, observe and record wa veform for the input signal, , output of stage 1, , and output of stage 2, . Show amplitude and phase relations clearly.
