TELE-TECH & - AmericanRadioHistory.Com

By W. F. CHOW
Meter
Fig.
3: Input circuit. To supply constant power V, must vary as the square root of R,
tive as well as purely resistive, depending
on the circuit connection
and the frequency. The exact input
impedance match will require the
conjugate complex impedance for
the signal generator. When the input
impedance varies with frequency,
this conjugate complex impedance
should also change with frequency.
Theoretically it is not impossible to
achieve this aim, but from a practical
point of view the complication of
components involved for an exact
impedance match precludes this approach.
If a pure resistance r,, is used
for the signal generator impedance,
the amount of available input power
is (VF)2 /4rF, where V is the open
circuit voltage of the signal generator.
A small amount of error is introduced
in the value of the output
power due to the absence of complex
match of the input impedance.
This error will be discussed later.
If the available signal power (VF)2
/4r is used as the input power of the
transistor amplifier, then the source
resistance rF should be variable
while the available input power
should remain constant. This demands
a generator with variable internal
resistance but constant available
power. The signal generator
available in the laboratory is usually
of the constant voltage type. The
method of converting the constant
voltage generator to a constant
power generator is as follows. In
Fig. 3. the constant voltage is connected
to a number of resistances in
series. A resistance rF in series with
the load is connected to the circuit as
shown in Fig. 3. If the available
power feeding to the load from the
source on the left side of the line a-
a will be (VF)2 /4rF. In order to keep
the available power constant when
r,, varies, the value of VF has to vary
accordingly, that is, VF has to vary as
the square root of the variation of rF.
For example, if rF increases 2 times,
the value of V_, has to increase \'2
times. This increase of VF can be easily
achieved by moving the tap of rg
up so that the voltage across r, is
increased V/2 times.
On the output side if pure resistances
are used as loads, a certain
amount of error due to not using
conjugate complex impedance match
is introduced. This error is small
only at low frequencies. Therefore in
general, a resistance r,, together with
a parallel tuned tank circuit should
be used as the load impedance. The
Q of the tank circuit should be high
enough such that the loss is negligible
compared to the power delivered
to r1,. Since a power meter is not
available for all the frequencies, a
sensitive ac millivoltmeter may be
used to measure the power delivered
to r,,. This voltmeter should give the
same reading for the same amount of
power dissipated in the different values
of load resistance. Fig. 1 shows
a method of accomplishing this pur-
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Fig. 4: Converting the my readings to power
gain in db for input voltage of 100 my
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Fig. 5: Variation of available power gain
pose. The voltage drop across the
load r,, is monitored at a proper tap.
This tap will vary according to the
square root of the ratio of the change
in the load resistance. For example,
if r,, increases to 3 times the original
values, the value of V. has to increase
to \/3 times its original value.
In this way the voltmeter reading
can be calibrated for power gain in
db.
Circuit Design
The actual circuit design of the
power gain meter depends on the
required flexibility. The choice of
values of r, and r, in Fig. 3 is determined
by the input impedance of the
transistor at different frequencies.
For example, for a pnp junction
transistor the input impedance may
vary with frequency from 40 ohms
up to several hundred ohms in
grounded base configuration. In the
grounded emitter connection, this
input impedance may be as high as
several thousand ohms. Once the
value of rF is determined, the value
of r, is made much smaller than r,;.
The value of VF is then riot changed
significantly by putting 2r, in parallel
with r,. Two continuously variable
resistances are desired for rF
and r1. They should be coupled mechanically
to give proper resistance
values.
The value of the load impedance
is determined by the output impedance
of the transistors. For example,
a pnp junction transistor operated
as a grounded base amplifier
may have an output impedance of
several hundred kilo -ohms and
higher at low frequencies falling to
several kilo -ohms as the frequency
increases. (The effective shunt reactance
of the output impedance is
normally capacitive. This reactance
is high at low frequencies. Therefore,
the output impedance can be considered
as resistive with small error.
(Continued on page 355)
Tele -Tech & ELECTRONIC INDUSTRIES June 1956
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