OBJECTIVE:
APPARATUS:
INTRODUCTION:
For an npn transistor the collector is positive
relative to the emitter.
The base-emitter circuit acts like a diode and is normally conducting
(i.e. forward-biased). The base-collector circuit also acts as a diode but is
normally non-conducting (reverse biased) if no current flows in the
base-emitter circuit. However when current
flows in the base-emitter circuit, the high concentration gradient of
carriers in the very thin base gives an appreciable diffusion current to the
reversed biased collector.
The resulting
collector current Ic depends on the base current Ib. We write
I
c = Ib where
is the current amplification factor.
SUGGESTED EXPERIMENTS:
1. CURRENT Amplification: Measure |
Figure 2
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Repeat the readings for ten reasonably spaced (higher) settings of the voltage divider (i.e. higher Veb and hence higher Ib.
Calculate
(= Ic/Ib) and plot it against Ib. Over what range of
Ib is
reasonably constant?
Repeat the above but with emitter to collector voltage Vec now at 7 V.
Graph Vout vs Vin and calculate the voltage gain G at the steeply changing
part of your graph. The value of Vin at the center of this region we
call the ``operating voltage'' of the amplifier.
How would the gain change if the load resistance was
2.2 k
instead of
10 k
?
Adjust the voltage divider until the Vin is close to the ``operating voltage'' found in #3.
Vary (and record) the amplitude of the input signal from small values to those which overdrive the amplifier and produce considerable distortion in the output Y2.
Observe and record the effect of changing Vin
to values outside of the
operating range (where is constant).