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 RBB, the intrinsic stand-off ratio Ƞ and peak-point voltage VP, as defined in UJT can be programmed to any desired values through external resistors RB and RB2 and the supply voltage VBB. From figure we see that by voltage divider rule when IG = 0,
VG = (RB1 / RB1 + RB2 ) VBB = Ƞ VBB
Consider figure The P-N-P-N device shown in figure has its gate connected to the junction of external resistors RB and RB . 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 VAK 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.
The typical characteristics of the device are shown in figure. The firing or peak-point potential is given as
VP = Ƞ VBB + VB as defined for the UJT.
However VP represents the voltage drop VAK in figure [ the forward voltage drop across the conducting diode]. For silicon VB is typically 0.7 V.
In PUT RB1 and RB2 are the external resistors to the device permitting the adjustment of Ƞ and hence VG while in the UJT both RB1 and RB2 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, 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 VBB is switched on, the capacitor starts charging toward VBB volts, since there is no anode current at this point. The instant the voltage across the capacitor equals VP, the device fires and anode current IA = IP 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 RK. Consequently, a voltage spike is produced across RK 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 VP is given approximately by the expression
T = RC loge = VBB / VBB – VP = Ƞ VBB
At the point of firing of PUT
IP R = VBB – VP
If R is too large, the current IP cannot be established, and the device will not fire
So RMAX = VBB – VP / IP
Similarly RMIN = VBB – VV / IV
VBB – VP / IP > R > VBB – VV / IV