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Experiment 1

Multistage Amplifier – Part 1

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Objectives:

Investigate the multistage amplifier design using simple BJT amplifier circuit, especially cascade and cascade connections practically and by simulation

Prelab:

Consider the circuit shown in figure 2 and using OrCAD Pspise:

  1. Connect the first stage amplifier without AC input (only DC), then find and plot 1st transistor Q-point (Ic and Vce).
  2. Like one, find and plot the Q-point of the 2nd transistor.
  3. Now connect the multistage circuit with sinusoidal input (50mV, 1 kHz).
  4. Plot the input voltage, 1st stage output voltage, 2nd stage voltage. " Don't plot the signals on the same graphs".
  5. Let the output of the first stage on the emitter (VE1), the second output on the collector and then plot the input voltage, 1st stage output voltage, 2nd stage voltage. " Don't plot the signals on the same graphs".
  6. Let the output of the first stage on the emitter (VE1), the second output on the emitter (VE2) and then plot the input voltage, 1st stage output voltage, 2nd stage voltage. " Don't plot the signals on the same graphs".
  7. Comment on your results at (4, 5, and 6).

Theoretical Background:

The properties of BJT amplifiers can be summarized in table(1).

These basic amplifier stages can be combined by cascade connection or cascode connection to create multistage amplifiers with better overall characteristics.

  • Cascade connection :

In which each stage is coupled with the next by a capacitor. Figure (1) shows a general model for a cascade multistage amplifier configuration.

Emitter follower CC / CE with RE / Common emitter
/ / / Construction
Low / Medium / High / Voltage gain
High / High / Low / Input resistance
Low / High / High / Output resistance

Table (1) : Properties of BJT amplifiers

Figure (1) : General model for a cascade multistage amplifier

The gain of multistage amplifier is the product of the individual gain stages. For the generalized example in figure (1), the over all gain can be calculated as :

Av(total) = Av1 . Av2 . Av3 ------(1)

The input resistance of the multistage amplifier equals to the input resistance of the first amplifier stage as following:

Rin(total) = Rin1 ------(2)

And the output resistance will be

Rout(total) = Rout3 ------(3)

These properties will help us to design a multistage amplifier that has high gain, high input resistance and low output resistance by carefully combining the basic BJT amplifier stages.

  • Cascode connection :

In which the collector of the leading transistor is connected to the emitter of the following transistor as shown at figure (3).The cascode connection offers high gain, high stability, and high input impedance.

Lab Work:

For the circuit shown in figure (2) :

  1. Evaluate the DC Q-point for each transistor.
  2. Connect the multistage circuit, plot the output voltage using (50 mV, 1k Hz) sinusoidal input. Then determine the voltage gain.
  3. Let the output of the first stage on the emitter, plot the output voltage and determine the voltage gain.
  4. Let the output of the second stage on the emitter, plot the output voltage and determine the voltage gain.
  5. Comment on your result.

Figure (2) : Cascade multistage circuit

Exercises :

  1. Calculate the DC Q-points for each transistor shown in the circuit in figure (2), assume that β =100
  2. Derive Zin, Av, Zout for the circuit in figure (2).
  3. Simulate the circuit shown at figure (2) and figure (3).

Figure (3) : Cascode multistage circuit