-8-
Transistor Q5 is called the pass transistor. It controls the current reaching the output. Transistor Q3 and Q4 are
emitter followers. Their function is to raise the impedance of the pass transistor. Note that transistor Q2, Q3, Q4,
Q5 and resistor R1 form a close loop. Also note that the feedback to the base of Q2 is negative, that is, when
the base of Q2 goes positive, the output at emitter Q5 goes negative. Now if the 2V output voltage goes down
because of current drain at the output, the base of Q2 will drop forcing the the collector voltage of Q2 to go higher.
This will bring the output voltage back to 2V. This is the basis of all negative feedback regulators.
Another feature of the LM-317 regulator is to protect the IC against overload and output shorts. If the IC is
overloaded, the junction of an overload transistor will overheat. A transistor will sense this overheating and shut
down transistor Q5.
The LM-317 IC is basically a 1.25 regulator. To be able to vary the output
1.25 to 15V we stack the IC on a DC voltage as shown in Figure 6A. When
VR1 equals 0, the output voltage is 1.25V as determined by the LM-317 IC.
Note that the voltage across R1 is always 1.25 volts. When R1 equals VR1,
the voltage across VR1 will equal the 1.25 volts across R1, therefore the
output voltage will be 2.5 volts. When VR1 is 5 times R1, the output voltage
is 6.25 volts. As you can see, varying resistor VR1 will vary the voltage from
1.25 to 15V.
THE NEGATIVE VOLTAGE REGULATOR
The theory of the voltage regulator is the same as the previously discussed positive regulator. The basic
difference is that diodes D1 and D2 are reversed producing a negative voltage across capacitor C1. The LM-337
IC is designed to operate from a negative supply.
THE DATA SWITCHES
There are eight data switches labeled “0” to “7”. The circuit is very simple. To perform the desired functions
there is a double throw-double pole switch. One end is connected to the 5V, the other to ground and the center
lug is connected to the output.
THE LOGIC SWITCHES
The logic switches perform the same function as the data switch, that is, they produce high or low states. But
there is one big difference. When switching the data switches, many pulses may be produced due to bouncing
of the contacts.
In the logic switches, only one pulse is produced, no matter how many
times the contacts bounce. This is extremely important if you are
producing pulses for counting circuits. Figure 7 shows the wiring of the
logic switch. The two NAND gates are connected so that when A input
is grounded the output A goes high. Opening and closing the ground
at A will not change the output. Only when A is grounded will the
output change to low. Thus, only one output change is produced with
one movement of the A switch. There are two outputs from logic
switch, A and A or B and B.
THE FUNCTION GENERATOR
The function generator frequencies are produced by an XR-2206 integrated circuit. This IC is capable of
producing high quality sine and square waveform of high stability and accuracy. Figure 8 shows the block
diagram of the XR-2206 IC.
Figure 6A
Figure 7
1.25 - 15V
R1
VR1
LM-317
A
A
A
A
6
5
4
1
2
3
(16 pages)
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