(i) In order that the triac is triggered only by a positive gate voltage, we can use the method shown in Fig. 21.15. In this circuit, diode D1 is forward biased when VGT is positive and reverse biased when VGT is negative. Since D1 will conduct only when VGT is positive, the triac can only be triggered by a positive gate signal. The voltage VA required to trigger the device is equal to the sum of VF for diode D1 and the required gate triggering voltage i.e. VA = VF + VGT = 0.7V + 2V = 2.7V