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Equipment
NOTES ON TECHNIQUE AND CONVENTIONS
Vcc
or Vdd
is supposed to be signal (AC) ground. To assure this, you should
always bypass power connections at the circuit with a capacitor
appropriate to the circuit. For audio frequencies, this usually
means a large(³10mF)
electrolytic.Schematic diagrams usually omit this capacitor. In
making connections to test equipment, take account of input and
output impedances. Remember that coaxial cables are low impedance
for short times; for long times, they look like capacitors of
about 30pf/foot. Short refers to travel time in the cable, or
about 1.4 nanoseconds/foot.
THE GENERIC BIPOLAR AMPLIFIER CIRCUIT
The circuit above can be easily converted
into all three of the possible amplifier configurations. If RE
is bypassed, the amplifier becomes CE (common emitter). The input
is at terminal #1 and the output at terminal #2. If Rc
is shorted or bypassed, it becomes CC and the input is at terminal
#1 with output at #3. If R2
is bypassed it becomes CB with input at #3 and output at #2.
We can not actually short the above terminals without altering
the DC biasing that sets the operating point. However, if the
capacitors, C1
, C2
, and C3
are large enough, they can be used as AC short circuits. After
you select values for the resistances as discussed in the next
section, then decide how large each of these capacitors must be
in order to function in this way assuming a signal frequency of
10 kHz. Because of the large capacitance required, electrolytic
capacitors are commonly used at audio frequencies.
Setting the operating point.
Use your bench power supply to provide VCC= 12 V. Use a self bias resistor RE Å 300 to 1000 W. From your previous measurements of the 2N5210 you should know that a reasonable operating current is 1 to 2 mA. In order to allow a usefully large voltage swing on the output you would like to have VCE Å0.6 VCC. Choose your value of RC accordingly. Since VBE Å 0.6 V, you can now determine the desired VB and from this the desired ratio of R1 / R2. What is a reasonable criterion for the actual values of these resistors?
Measure each resistor that you select for
the circuit to make certain that you have close to the desired
value. Then assemble the circuit on your proto-board. Measure
and adjust the power supply before connecting it to the circuit.
With the circuit operating, use the DVM to check the voltages,
VCE
, VE
, VB
. If they are not as desired, change the appropriate resistors
to make them so.
Compute and measure the AC gain and its frequency dependence
For each of two circuit configurations (refer to Figure 1): 1) C3 connected to ground (CE), 2) C2 connected to ground (CC or emitter follower), use the small signal model to calculate the gain expected from the circuit with the values of the resistors you have used. Then apply a 10 kHz sine wave, between 0.01 and 0.1 V peak to peak, to the input and monitor both input and output on the oscilloscope to measure the gain. Determine the range of signal amplitudes over which the response is linear by increasing the input voltage until the output voltage is distorted. You will find this can be done very effectively by superimposing the two signals on the scope screen with their relative gains adjusted so as to produce exactly the same trace on the scope.
Increase and decrease the frequency of the
input signal until the gain is decreased by 1/Ã2. Record
the frequencies of these half power points and estimate the phase
shift between input and output from the two traces on the scope.
In which configurations do you expect a 180o phase shift in the
mid frequency range? Calculate, from the component values used,
the low frequency at which the gain drops to 1/Ã2. The high
frequency half power frequency is due to the internal design of
the transistor.
Measure the input and output impedance
For the same two circuit configurations, measure
the input and output resistance at 10 kHz by connecting a resistance
in series with the input and then in parallel with the output.
The decade resistance box is very useful for this purpose since
it allows you to find exactly the resistance at which the input
or output signals are reduced by one half. Describe this process
and the results. Do not use the definition Zout=V(open
circuit)/I(short circuit). You lack a good way to measure AC current,
and some circuit configutations (CE ) cannot drive zero impedance.
THE GENERIC JFET AMPLIFIER
A general purpose circuit for an JFET amplifier,
analogous to the bipolar transistor amplifier of figure 1, is
shown in figure 2. This circuit takes advantage of the very high
input impedance of the JFET as long as a large value resistor
(say 1 megohm) is used for RG.
Setting the operating point
Explain the self biasing scheme of this circuit.
Again use a supply voltage VDD=12V.
Assume IDS
1 mA and chose RD
so as to allow about a 6 V swing for the signal. Now use the
decade resistance box for RD
and adjust it so that VD
Å6 V.
Measure the gain and its frequency dependence, and the input and output impedance
For the common source, and common drain (source
follower) configurations measure the voltage gain at 10 kHz and
find the high and low frequency half power points. Measure the
input and output impedance. What value of gm
does your measured gain correspond to?
Is its value consistent with other typical JFET's? Does the
gain of the amplifier increase in proportion to RD
as in the small signal model?