For circuit designs that use complementary
you sometimes need to sort the NPN
and PNP transistors to have matching
dc-current gains (β). One example of a
circuit requiring matching is the output
stage of an amplifier. The circuit in
Figure 1 shows a simple test fixture to
achieve this match.
Figure 1This circuit makes it easy to test and match the current gain of complementary bipolar transistors. Matched transistors will cause the voltmeter to read 0V.
are the devices
being tested to see if they are matched. In
the test fixture, Q1
share the same
base current (IB
); since there is no additional
path where the current can flow,
no additional compensation is needed.
Note, however, that β should be high
enough that IE
. With this detail in
mind, resistors R1
should be equal.
To give the transistors a bit more
headroom, an additional voltage drop
is introduced between the transistors’
base connections. A voltage differential
of a few volts is desirable, so a blue
LED is a good choice for D1. Its presence
helps to set the base voltage for Q1
(VB1) to about half of the supply voltage
(VS). Using an LED in the place of
D1 is preferable to using a zener diode
due to the sharper knee at the low currents.
Moreover, you can see the glow
of many blue LEDs at currents below 10
μA; the glow indicates the presence of
base current, which means the circuit is working properly. Equation 1 is used
to determine the needed supply voltage:
A typical blue LED will have a forward
voltage of about 3.5V; assuming
VBE1=VBE2=0.7V, you get a value for VS
of about 9.8V.
Resistor R1 sets the emitter current
of Q1; it is calculated using Equation 2:
Figure 2 For a simpler version, replace
the voltmeter with inverse-parallel-connected
You should select an emitter current
that matches the application in which
the transistors will be used, because beta
varies with emitter and collector current.
With matching transistors (β1
) installed in the test fixture, the voltage
drops across R1
are equal, and the
voltmeter will show 0.
The circuit in Figure 2 is functionally
equivalent but uses a simpler method
to indicate when the circuit is in balance.
With matched gains, neither of
the red LEDs (D2 and D3) will be on.