# Bias current for voltage amplifier

Discussion in 'Electronics Homework Help' started by kbcheong, Apr 1, 2012.

1. ### kbcheong

25
0
Mar 24, 2012
Diagram attached showed a circuit for a voltage amplifier. Assume that the power supply voltage VDD =100V, and the resistances Rs = 22.0 ohm, and RL = 120 ohm, K = 2.0 A/V^2, VT = 2.0V. If the input bias voltage VI is 6.5, what is the bias current ID in Amperes?
May I enquire the formula for bias current?

File size:
12 KB
Views:
154
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,839
Jan 21, 2010
Maybe you should use the discussion forum provided for you as part of 6.002x?

11,646
2,691
Nov 17, 2011
4. ### Laplace

1,252
184
Apr 4, 2010
To find the bias point for this circuit requires two equations. There is the characteristic equation for the MOSFET relating VGS to drain current, and the bias equation relating bias voltage to drain current.

ID=(K/2)(VGS-VT)^2

VI=VGS+IDxRs

Note that the characteristic equation is symmetrical around VT but is only valid for VGS>VT. Plotting the bias equation gives the bias line on the graph. Where these two plots intersect is the bias point. See attachment. If I did this right then the graph should show the same bias point you calculated.