Tunnel Diode Parallel Amplifier Circuit:

For operation as an amplifier, a tunnel diode must be biased to the center of its negative resistance region. Figure 21-17(a) shows the basic circuit of a Tunnel Diode Parallel Amplifier Circuit.

Load resistor R_L is connected in parallel with diode D₁ and supplied with current from voltage source E_B and signal source e_s. Figure 21-17(b) uses the tunnel diode piecewise linear characteristics to show the dc conditions of the diode when the signal voltage is zero (e_s = 0), and when e_s = ±100 mV.

The current gain equation for a Tunnel Diode Parallel Amplifier Circuit can be shown to be,

Note that R_D is already taken as negative in Eq. 21-2, so that only the absolute value should be used in calculating A_i. For R_D = 100 Ω and R_L = 80Ω,

From Eq. 21-2, it is seen that (when R_L ≪ R_D, A_i ≈ 1), (when R_L ≫ R_D, A_i < 1), and (when R_L = R_D, A_i = ∞). A current gain of infinity means that the circuit is likely to oscillate. For maximum stable current gain, R_L should be selected just slightly less than R_D.

Figure 21-18 shows the circuit of a practical tunnel diode parallel amplifier. The signal voltage e_s and load resistor R_L are capacitor-coupled to the diode, while dc bias is provided by source voltage E_B and voltage divider R₁ and R₂. Inductor L₁ and capacitor C₁ isolate the bias supply from ac signals.

A tunnel diode series amplifier can be constructed. In this case the device is connected in series with the load, and voltage amplification is obtained instead of current amplification. Oscillators and switching circuits can also be constructed using tunnel diodes.