Electronic Circuits and Diagram-Electronics Projects and Design

Programmable UJT

The programmable unijunction transistor (PUT) is not a unijunction transistor at all. The fact that the V-I characteristics and applications of both are similar prompted the choice of labels.

It is also a four-layer P-N-P-N solid-state device with a gate connected directly to the sandwiched N-type layer. The basic structure, schematic symbol and the basic biasing arrangement of PUT are shown in figures respectively. As the symbol indicates, it is essentially an SCR with a control mechanism that permits a duplication of the characteristics of the typical SCR. The term “programmable” is applied because the inter base resistance R_BB, the intrinsic stand-off ratio Ƞ and peak-point voltage V_P, as defined in UJT can be programmed to any desired values through external resistors R_B and R_B2 and the supply voltage V_BB. From figure we see that by voltage divider rule when I_G = 0,

V_G =   (R_B1 / R_B1 + R_B2 )  V_BB =  Ƞ  V_BB

Consider figure The P-N-P-N device shown in figure has its gate connected to the junction of external resistors R_B and R_B . The four-layer construction shown in figure indicates that the anode-gate junction is forward biased when the anode becomes positive with respect to gate. When this occurs, the device is turned on. The anode-to-cathode voltage V_AK then drops to a low level, and the device conducts heavily until the input voltage become too low to sustain conduction. It is seen that this action stimulates the performance of a UJT. The anode of the device acts as the emitter of UJT.

Characteristics of Programmable UJT

The typical characteristics of the device are shown in figure. The firing or peak-point potential is given as

V_P = Ƞ V_BB + V_{B  as defined for the UJT.}

However V_P represents the voltage drop V_AK in figure [ the forward voltage drop across the conduct­ing diode]. For silicon V_B is typically 0.7 V.

In PUT R_B1 and R_B2 are the external resistors to the device permitting the adjustment of Ƞ and hence V_G while in the UJT both R_B1 and R_B2 represent the bulk resistance and ohmic base contacts of the device (both inaccessible).  Although the characteristics of the PUT and UJT are similar, the peak and valley currents of the PUT are typically lower than those of a UJT of a similar rating. In addition, the minimum operating voltage of PUT is also lower than that of UJT.

Application of PUT

PUT RElaxation Oscillator

PUT, because of its superiority over UJT, replaces UJT. One popular application of PUT is in the relaxation oscillator shown in figure. The instant the supply V_BB is switched on, the capacitor starts charging toward V_BB volts, since there is no anode current at this point. The instant the voltage across the capacitor equals V_P, the device fires and anode current I_A = I_P is established through the PUT. As soon as the device fires, the capacitor starts discharging rapidly through the low on-resistance of the PUT and R_K. Consequently, a voltage spike is produced across R_K during the discharge. As soon as the capacitor C gets discharged, the PUT turns off and the charging cycle starts all over again as narrated above.

The time period required to attain the firing potential V_P is given approximately by the expression

T = RC log_e = V_BB / V_BB – V_P = Ƞ V_BB

At the point of firing of PUT
I_P R = V_BB – V_P

If R is too large, the current I_P cannot be established, and the device will not fire

So R_MAX = V_BB – V_P / I_P

Similarly  R_MIN = V_BB – V_V / I_V

V_BB – V_P / I_P > R > V_BB – V_V / I_V