Transistor Switches Voltage Regulators
Of the four regions for transistor operation discussed in the last
lecture, only cutoff and saturation play important roles when the
transistor is used as a switch. In the NorCal 40A, Q1 is used as an npn transistor switch in the Receiver (RX) Switch circuit.
The operation of the RX Switch can be summarized in two
1. When the key is “up,” the transmitter is off so 8V TX = 0.
Consequently, IB = 0 and Q1 is “off,” i.e., in the cutoff
mode where IC = 0. Current through C1 continues to L1
since the switch Q1 is “off.”
2. When the key is “down,” the transmitter is on so 8V TX ?
8 V. In this state, we want Q1 to be completely saturated so
the resistance seen from collector to emitter ? Rs ? 0.
This state is achieved when IB and IC are large enough to
not only forward bias the EBJ, but also the CBJ. This is
called saturation. When saturated, Rs ? 2? for the 2N4124 used in the RX Switch. Furthermore,
Consequently, Rs is very small wrt XC1 and XL1. Therefore, Q1
operates as an effective short to ground. (This is necessary to
keep the transmitted signal from entering the receiver circuit
when the NorCal 40A is transmitting.) The actual value of the saturation resistance Rs is highly dependent on IB. See Fig. 8.6 of the text for measured values from the 2N4124. pnp Transistor Switches
The npn transistor makes a good short-to-ground switch. The
pnp BJT is also used as a transistor switch, but often to connect
a voltage source to a load. In the NorCal 40A, an example of this type of switch is the Transmitter (TX) Switch
The operation of the TX Switch can be summarized in two
1. With the key up, Keyline is open circuited and C57 is fully
charged to 8 V through R24. Hence, IB = 0 and Q4 is
cutoff. Therefore, VC = 0 and the transmitter circuitry is not
2. With the key down, Keyline is short circuited to ground
and C57 discharges through D11 (down to the forward bias
voltage of D11). Current flows in R9 and IB.
We design this circuit so that IB is large enough to saturate
Q4. Then, with Vvc|sat ?0.2 V, Vc ?7.8 V and the transmitter circuitry is energized. In this state (key down), we have successfully connected a voltage source to a load. Design of the Transmitter Switch
Our task now is to design the Transmitter Switch circuit to
completely saturate Q4 when the key is down. To do this, use
Fig. 14 of the 2N3906 datasheet (p. 377). This figure shows VCE
vs. IB for families of IC. First, let’s estimate IC when Q4 is saturated:
From this circuit, of the datasheet you’ll find an IC = 10 mA curve. In our circuit, IC will be no larger than this. Hence, from this IC = 10 mA curve:
V vc=0.2 V with IB ?140 ?A.
Consequently, we need to design the TX Switch circuit so that at
key down, IB ?140?A.
Note that for this IB and with IC < 10 mA (say 7 mA as we have
estimated), then Q4 is driven “deeper” into saturation. So we
have a built-in safety factor using the IC = 10 mA curve in the
When Q4 is saturated, IC = ? IB . Rather,
?min is specified in transistor data sheets. This is one way to test
if a transistor is saturated. You’ll complete this design and measure the results in Prob. 20. Keying Relay
In order to make time constant and other measurements in
Probs. 20, 25, and 30, you’ll need to turn the key on and off
relatively quickly and repeatedly.
In these problems, you will use an electromechanical relay since
turning the key on and off by hand will not be practical.
The relay you’ll use is the W171DIP-7, which is DIP package
like an IC. Very cool
With V “high,” the relay closes. The diode serves as a snubber
diode, as we saw earlier in Here are connections you can use to J3 from the AWG in Prob.20.
Also in Prob. 20, you will install U5, which is a 78L08 voltage
regulator. You don’t need to know much about voltage
regulators in the assembly and test of your NorCal 40A.
However, we will quickly summarize this IC to provide relevant
The 78Lxx series of ICs are three terminal devices that provide
fixed dc output voltages, typically with currents no greater than
100 mA. Available voltages include 4, 5, 6, 7, 8, 9, 10, 12, 15,
18, 20, and 24 Vdc (among others!).
In the NorCal 40A, you are using a 78L08 which provides 8
Vdc. This voltage is used by all of your receiver and much of the
transmitter circuit, except for the Driver and Power Amplifiers.
The datasheet for the AN78Lxx series (Panasonic) of voltage
regulators is found on the EE 322 web page, while Appendix D
of your text includes the datasheet for the MC78Lxx series
These voltage regulator ICs must supply a constant (or nearly
constant) output voltage as:
1. the input voltage changes. This is called line regulation.
2. the load connected to the regulator changes. This is called
3. the temperature changes (these devices generally heat up).
Here the relevant specifications for the AN78L08:
In the NorCal 40A, the function of the 78L08 is to provide 8
Vdc from a source voltage ranging from 10-15 Vdc.
Consequently, the line regulation specification is not too
important here since we’re converting from dc to dc.