Problem:

A DC motor must be driven from a 5V power supply with bidirectional control over the motor shaft. An H-bridge circuit is desired, but unfortunately you only have 4 NMOS transistors available (instead of 2 NMOS and 2 PMOSas desired for a proper H-bridge). Given that the transistors have a threshold voltage of VT= 2V and µnCox(W/L) = 0.5 A/V2, do the following:

Required:

a) Draw a full circuit diagram of the H-bridge circuit using the 4 NMOS transistors.

b) At a particular value of VGS, a saturation current of I_DSAT = 0.6 A is determined for the transistor. Find this value of VGS. (this has nothing to do with the H-bridge, yet)

c) If a maximum motor current of 0.6 A is desired when operating at 100% duty cycle, draw a table showingthe input voltages (V1 through V4) to each transistor required to generate forward and reverse motion. Use the lowest possible voltage in each case. You may exceed the power supply volage for your gate voltages if necessary. Number your transistors using the same convention as in the lecture notes.

d) From the above results, summarize a basic argument why PMOS transistors are prefered for high-side drive.