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By | June 2011
Page 1 of 3
Experiment:

The main setup of the circuit was provided in the lab manual. There were basically three stages in the circuit. The first one consisted of a Voltage follower with FET. The second stage consisted of an integrator and the third stage was a Schmitt trigger. The output of the Schmitt trigger goes through a switching mechanism that changes the function of the voltage follower. Theoretically, our input was 0 to 10 volts but due to lack of equipment the input was only taken to a max of 6 volts from the DC supply. The 20 V power supply was in use by the OP-AMPS for their power. The following stages of the voltage to frequency converter are now explained in details.

Voltage follower with FET :

The basic purpose of the Voltage follower is to give a Unity gain and follow the input voltage to the integrator. At its input from the Schmitt Trigger its connected to a FET which plays a very important switching role. Figure 1 describes the circuit used in the experiment.

Figure1.

FET ON:

When the FET is on, the resistor R12 is grounded which according to the following calculations gives Vo equal to – Vin.

(Vin-Va)/R13 = (Va-Vo)/R14 -------------------------1

Since Va=0 because of the FET provided a short circuit to the ground, equation 1 becomes:

Vin/R13 = -Vo/R14

Vin = -Vo R13/R14

R13 = R14

Vin = -Vo

FET OFF:

When the FET is off the Q1 is opened giving V (output) equal to Vin according to the following analysis:

(Vin-Va)/R13 = (Va-Vo)/R14

VinR14 – VaR14 = VaR13 – VoR13 ----------------------------2

Va= Vin both the resistors R11 and R12 are dead because no current passes through them. Equation 2 becomes :

VaR14 – VaR14 = VaR13 – VoR13

0 = VaR13 – VoR13

Va = Vo

When input voltage is positive, the gate voltage would be the voltage drop of the diode. The switch will be OFF because Vgs < Vt. On the other hand, if the input voltage is negative then the switch will be ON, because Vds < Vgs...