TM 9-6625-1753-14
(c) The input to the other half of the difference amplifier, V8202, is a dc Voltage determined by the setting of the
values of forward bias, the tunnel diode again functions as a conventional diode. The negative-resistance characteristic
LEVEL, control. When the point on the positive-going slope of the sine wave is equal to the dc voltage, the difference
(increasing the forward bias to cause a decrease in current) enables the tunnel diode to amplify, oscillate, and function as a
amplifier is balanced and it produces a constant out put. When the positive-going slope of the sine-wave signal exceeds the
switch.
dc voltage, the difference amplifier becomes unbalanced. In this state, V8201 conducts, producing a negative-going output
which goes to the cathode of tunnel diode CR-8201, forward biasing it. When the positive-going slope of the sine-wave input
(c) How the tunnel diode functions as a switch (this is how it operates in the trigger circuit) is discussed below. In
is below the dc voltage, the difference amplifier is also unbalanced. In this state, V8202 conducts more than V8201, so that a
figure 3-3B, a load line has been chosen that intersects the two positive regions of the curve at points A and D. These points
more positive voltage is applied to the cathode of the tunnel diode, reverse-biasing it. When the difference amplifiers are
represent the tunnel diode's two stable states: a high current, low voltage state, and a lower current, high voltage state.
unbalanced they cause the tunnel diode to produce a trigger pulse.
(d) Assume that the tunnel diode is forward-biased at point A (this point represents the setting of the LEVEL
(2) Tunnel Diode.
control). If a further negative potential is applied to the tunnel-diode cathode, the current increases and it switches to high-
voltage point C. When the current no longer increases (stabilizes), the biasing point falls back from C to D (the low current,
(a) The tunnel diode is a voltage-stable semiconductor device that exhibits a negative resistance over a section
high voltage stable state) on load line RL.
of its forward-bias characteristic. Figure 3-3A shows a typical tunnel-diode characteristic curve. As shown on the curve, the
tunnel diode is highly conductive and presents an extremely low impedance when it is reverse-biased. The reason the tunnel
(e) To bring the tunnel diode back to the low voltage, high current point (initial stable state), the biasing current is
diode is highly conductive when it is reverse-biased is because the valence electrons of the semiconductor atoms tunnel
momentarily reduced to a value less than the valley current by a positive potential applied to the cathode. The tunnel diode
across the PN junction from the P-type region to the N-type region. In the same way, when a small forward bias is applied to
thus switches to point F and then up to point A, the starting point (also on load line RL).
the diode, the electrons in the N-type region tunnel across the junction to the P-type region and the current increases rapidly
in the other direction to a maximum peak, Ip.
(f) Returning to the difference amplifier circuit, notice that TRIG SENS resistor R8229 is connected across the
tunnel diode. With this circuit configuration you can vary the current through the tunnel diode. You adjust the TRIG SENS
resistor to set the current through the tunnel diode to its optimum operating point (as close to point Ip as possible).
(g) TRIG BAL resistor R8234 (zone B6) sets the voltage level (reverse bias) that the difference-amplifier negative
output must overcome to forward-bias the tunnel diode. You adjust the TRIG BAL, resistor while LEVEL control R8222 is set
at zero. When the TRIG BAL resistor is properly adjusted, the SLOPE switch can be changed from plus to minus; the output
of the difference amplifier will still overcome the reverse-bias voltage level without readjusting the LEVEL control.
(h) Inductor L8201, connected in series with the TRIG SENS resistor and in parallel with the tunnel diode,
provides the high-impedance load required of the tunnel diode. The inductor increases the pulse amplitude.
(3) Shaping. The output of the difference amplifier and tunnel diode goes to differentiating amplifier Q8202. This
stage differentiates (shapes) the signal, converting it into a sharp trigger pulse which is applied to transformer T8201 (zone
A7). After passing through trigger diode CR8203, the negative-trigger output of T8201 goes to normal-sweep gate
multivibrator Q8206, Q8207 (par. 3-13).
e. Automatic Triggering.
(1) From the description in d above, you can see that an input signal is used to generate a trigger pulse. However,
there are occasions where it is desirable to produce a trigger pulse without an internal signal. When the automatic trigger
circuit is employed, a reference trace is displayed on the CRT, regardless of whether a signal is applied to the oscilloscope or
not.
(2) Placing the NORM-AUTO switch to AUTO activates the automatic trigger circuit. Automatic trigger amplifier
Q8201 (zone B5) is connected between the output (plate of V8201) and input (grid of V8202) of the difference amplifier. RC
network R8211, C8207, in the collector circuit of Q8201, generates a triangular waveform which is applied to V8202. Using
AUTO SYNC ADJ resistor R8210, you can make the triangular signal symmetrical as well as set its frequency to about 50
cycles.
f. Delayed Trigger Generation. The delayed trigger circuit is similar to the normal trigger circuit; therefore, the
description given in d above will apply to this circuit as well.
Figure 3-3. Tunnel diode characteristic.
(b) As the forward bias increases to various intermediate values (from Vp to Vv), the current decreases to a deep
minimum valley at point Iv. This represents the important negative resistance characteristic of the tunnel diode. At higher
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