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Power Gain Meter
(Continued from page 105)
At high frequencies, the output impedance
may have a phase angle of
45° and higher. When a parallel
tuned resonant tank is used, the effective
capacitance of the output impedance
is in parallel with the condenser
of tank circuit. For a given
frequency, the effective capacitance
of the output impedance determines
the maximum value of inductance of
the tank circuit.) Two continuously
variable resistances are desirable for
r,, and r2 in Fig. 1, and proper mechanical
coupling is required.
Another point which demands attention
in the circuit design concerns
the dc bias connections. These
paths should, if possible, have a high
impedance for the ac signal, and low
resistance for dc current. The blocking
condenser in the ac path should
have a capacitance large enough to
let the lowest frequency encountered
pass through without too much attenuation.
This point is especially
important on the input side of this
instrument.
The circuit described below is a
power gain meter which can be used
for pnp and npn junction transistors.
This equipment in conjunction with a
common laboratory signal generator
and an ac millivoltmeter covers a
frequency range from 10 xc to 3 Mc.
In order to cover the radio frequencies
without introducing too much
error, the continuously variable resistances
r,, rg r, and r,, described
above are replaced by gauged
stepped controls. The error introduced
will be discussed later. The
complete circuit is shown in Fig. 3.
In this circuit the switch S2 is used
for changing the bias current when
the transistor circuit is changed from
the grounded -base to the grounded -
emitter connection. The switch S4 is
used to select the polarity of the
power supply for either pnp or npn
transistors. The center position of S4
is used to turn off the supply.
The total resistance presented to
the signal generator is 50 ohms.
When the switch SL is at position I,
r, is 5 ohms and rg is 50 ohms. If a
signal voltage of 100 mv is applied
across the 50 ohms, a Vg of 10 mv
appears across the 5 ohm resistor.
When a 50 ohms input resistance is
connected in series with rg, a total
of 100 ohms is now in shunt with r,
which is 5 ohms. A slight change of
Vg results. Let us neglect this small
change of vg for the time being and
(Continued on page 356)
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