Depending on the circuit configuration asso- 

 ciated with the tunnel diode, the device can 

 operate in a switching, amplifier or oscillator 

 mode. The amplifier mode, when used as a pres- 

 sure transducer, has a gage factor which can he 

 as high as 30,000. 2 Considering the bonded wire 

 strain gage with a gage factor of less than 10, 

 the potential of the tunnel diode as a pressure 

 transducer can be appreciated. The 15$ decrease 

 in peak current associated with germanium tunnel 

 diodes under 20,000 psi pressure is responsible 

 for the great sensitivity. 



fc bb r z 



TUNNEL DIODE PRESSURE TRANSDUCER OSCILLATOR MODE 



Conventional Oscillator Mode 



The series tunnel diode oscillator shown in 

 Fig. 2a and the series parellel tunnel diode 

 shown in Fig. 2b are the conventional oscillator 

 circuits used to generate a sine wave. The series 

 parallel oscillator shown in Fig. 2b is consid- 

 erably more stable than the series oscillator 

 because the "tank" circuit in the series parallel 

 configuration is primarily responsible for the 

 output frequency. Changes in frequency with pres- 

 sure would be small except at very high fre- 

 quencies where the tunnel diode shunt capacitance 

 is a large part of the "tank" circuit. 



The series tunnel diode oscillator shown in 

 Fig. 2a depends on the value of the negative con- 

 ductance for its frequency and stability. 5 The 

 negative conductance, -g^, is not single valued 

 and the value specified by the manufacturer is 

 an average one. Obviously, the slope of the 

 curve is voltage dependent; therefore, the sta- 

 bility of this configuration is not very good. 

 With pressure changes affecting the negative 

 slope of the I-E curve, stability is difficult 

 to maintain over a wide range. The change in 

 | g(j | due to the variation of the I-E character- 

 istic curve with pressure can be shown to vary 

 with the output frequency. Thus 



_1_ 



2TT 



f- 



-Br) |Sd| 



LC 



(1) 



where Rj is total equivalent circuit resistance, 

 L is series inductance, C is shunt capacitance 

 and |g(j| is the absolute value of negative con- 

 ductance. The requirement for stable oscilla- 

 tion is that L/C be made equal to Rip/|g, | . 



Hybrid Oscillator Mode 



The tunnel diode switching operation can be 

 understood by reviewing curve "A" in Fig. 1. 

 The peak and valley currents are the key switching 

 points . If the current reaches the peak value 

 the diode voltage increases to correspond with 

 the point "d" on the I-E curve. When the cur- 

 rent is reduced to the value of the valley cur- 

 rent the tunnel diode switches back to the low 



R T = r ' + * + r S + R DC («"0 



i r, t r 2_ 



(a.) SERIES 



C bb r z 



n£i 



i 



R = 



-g d f 



T ~V + |g d l z 



(b.) SERIES PARALLEL 



Fig. 2. Tunnel diode oscillator circuits 



voltage state. A current change is all that is 

 required to operate the tunnel diode switching 

 mode. This switching characteristic does not 

 depend on the value of g^. 



The hybrid configuration shown in Fig. 3 is 

 a relaxation oscillator with a square wave output. 

 The tunnel diode, when switching from the low 

 voltage state to the high voltage state, turns 

 the transistor, T-l, "on." Turning the transistor 

 on discharges the capacitance, Cj. When C-j_ has 

 discharged to the point where the current through 

 the tunnel diode switches back to the low voltage 

 state turning T-l "off, " C-^ then charges up 

 through the circuit provided until the current 

 through the tunnel diode reaches the value of the 

 peak current and switches the tunnel diode back 

 to the high voltage state. This turns the tran- 

 sistor, T-l, "on" and another cycle begins. 



The tunnel diode characteristic curve shows 

 that the values of the peak and valley currents 

 of the I-E curve depend on pressure. These in 

 turn establish the point of the charging or dis- 

 charging cycle where switching occurs . This 

 characteristic of the tunnel diode results in a 



190 



