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TransistorAmplifierCircuits /

Worksheet: Improved Transistor Amplifier

<^< Improved Transistor Amplifier | Course Index | 555 Timer Circuits >^>(:nl:)

Objective: To investigate the operation of a transistor amplifier with negative feedback bias.
Underpinning knowledge: Semiconductors. Transistors. Current gain. Amplifiers. Waveforms.
Apparatus: TINA circuit file, or apparatus to make the following circuit.

TINA Circuit File: Click here to download circuit file
Initial Settings: Input signal = 10mV peak, sine wave at 1kHz. Supply voltage = 10V. C'_1_' = 1mF. R'_B_' = 50k&#937;. R'_L_' = 1k&#937;.
Requires: Paper, Linear graph paper, pen or pencil.
Instructions:

(a) Use the oscilloscope to display two or three cycles of the voltage waveforms at the base and collector of the transistor. Sketch the waveforms and include axes.

(b) Measure, as accurately as you can, the peak-peak amplitude of the input (base) and output (collector) waveforms and use this to determine the voltage gain of the amplifier.

(c) Measure and record the voltage at the collector of the transistor with respect to common (0V). Note that, for optimum operation this voltage should be approximately half that of the collector supply (i.e. about 5V).

(d) Repeat steps (a) to (c) with R'_B_' = 25k&#937; and R'_B_' = 100k&#937;.

Observations:

Comment on the output waveform and voltage gain produced by the stage. What is the effect of increasing and decreasing the value of base bias current? How does the performance of this circuit compare with that of the simple transistor amplifier (WB25.CA4)?

Further work:

Replace the transistor with one or two common types of transistor using models available within TINA (you may have to make changes to R'_B_' and/or reduce the input signal amplitude to cater for different values of current gain). Determine the new gain.

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