Page 203 - Electrical Properties of Materials

The tunnel diode 185

I

(a) Rectifying current U

I

(b) Tunnelling current U

I

A

Fig. 9.25
The current in a tunnel diode is the
sum of the tunnelling current and of
(c) Total current U the usual rectifying current.

Consider an ordinary tuned circuit as shown in Fig. 9.26. If we start it oscil-
lating in some way, and then leave it, the oscillations will decay exponentially,
their amplitude falling with time according to
L C R

R
exp – t . (9.26)
2L
Physically, the resistance R absorbs the oscillating energy and gets a little hot-
ter. If we now put a negative resistance in series with R, odd things happen. In Fig. 9.26
A tuned circuit.
the particular case when the negative resistance (–R 1 ) is equal in magnitude to
R, the total resistance becomes
R – R 1 = 0, (9.27)

and the exponential becomes unity. This means that an oscillation, once started
in the circuit, will continue with no decay, the negative resistance replenishing
all the energy dissipated as heat in the real resistance.