13 Jul 2026

Characteristics of a Unijunction Transistor and Relaxation Oscillator

practical pg-ii ujt relaxation-oscillator

Aim

To study the emitter characteristic of a unijunction transistor and observe its use in a relaxation oscillator.

Apparatus

UJT, regulated DC supply, variable emitter supply, resistors, capacitor, CRO, voltmeters, and milliammeter.

Experimental arrangement

The capacitor charges through $R$ until the emitter reaches the UJT peak voltage, then discharges rapidly through the emitter-$B_1$ path to produce the pulse output.

Theory

A UJT contains a lightly doped n-type bar with ohmic contacts $B_1$ and $B_2$ and one p-type emitter junction. With the emitter open, the inter-base resistance is $R_{BB}=R_{B1}+R_{B2}$. Applying $V_{BB}$ establishes a potential at the emitter junction equal to a fraction of the inter-base voltage. The intrinsic stand-off ratio is

\[\eta=\frac{R_{B1}}{R_{B1}+R_{B2}}.\]

The emitter junction remains reverse biased until its voltage reaches the peak value

\[V_P=\eta V_{BB}+V_D,\]

where $V_D$ is the forward drop of the emitter junction. At the peak point, holes injected into the n-type bar reduce $R_{B1}$. The emitter current then increases while the emitter voltage falls, producing the negative-resistance region between peak and valley points.

In the relaxation oscillator, the capacitor charges through $R$ as $V_C=V_{BB}(1-e^{-t/RC})$. When $V_C=V_P$, the UJT turns on and the capacitor discharges rapidly through $B_1$. When the emitter current falls below the valley current the UJT turns off and charging begins again. Neglecting the short discharge time,

\[T=RC\ln\left(\frac{1}{1-V_P/V_{BB}}\right),\qquad f=\frac1T.\]

Observations

Emitter voltage $V_E$ (V)	Emitter current $I_E$ (mA)
0.5	0.0
1.0	0.0
1.5	0.2
2.0	1.4
1.7	3.0
1.4	5.0

The CRO shows a repeated saw-tooth waveform across the timing capacitor.

Graph

After the peak point, emitter current increases while emitter voltage decreases.

Calculation

The measured peak point is approximately $(V_P,I_P)=(2.0\,\text{V},1.4\,\text{mA})$. Using the peak and the last observed point, the mean dynamic resistance in the negative-resistance region is

\[r_d=\frac{\Delta V_E}{\Delta I_E}=\frac{1.4-2.0}{(5.0-1.4)\times10^{-3}}=-167\,\Omega.\]

The negative sign is the defining feature of this region.

Result

The UJT exhibits a negative-resistance region after the peak point and produces a saw-tooth relaxation waveform when connected with a timing resistor and capacitor.

Viva Questions

1. Why does the UJT show negative resistance? Carrier injection lowers the inter-base resistance after the peak point.
2. What determines the oscillator frequency? Mainly the timing resistance, capacitance, and peak voltage.
3. Why is the output saw-tooth shaped? The capacitor charges slowly and discharges rapidly through the UJT.

Maxima Code

Download the PG-II electronics calculation file.