operating principle of PUT.
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Constructional diagram & Operating principle of PUT:

image The PUT is a PNPN device similar to SCR, but its operation is similar to the UJT. The PUT behave like a UJT whose trigger voltage VP can be set by the circuit designer via an external voltage divider, so termed as “Programmable UJT (PUT)”. Fig. shows the PNPN structure and the circuit symbol for the PUT. The anode (A) and cathode (K) are the same as for any PNPN device. The gate (G) is connected to the Nregion next to the anode. Thus, the anode and gate constitute a P-N junction. It is this P-N junction which controls the “on” and “off” states of the PUT. The gate is usually positively biased relative to the cathode by a certain amount, Vg. When the anode voltage is less than Vg‟ the anode-gate junction is reverse-biased and the PNPN device is in the “off” state, acting as an open-switch between anode and cathode. When the anode voltage exceeds Vg by about 0.7V, the anode gate junction conducts, causing carrier injection at the middle reverse biased junction J2 and the PNPN device is turnedon. In the “on” state, the PUT acts like any PNPN device between anode and cathode (low resistance and VAK ≈ 1V). The PUT is also referred to as a complementary SCR (CSCR). 

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