enter image description hereI don't know how to ask the right question it would appear, but the following answer by Dave Tweed Refers:

Floating Voltage to Bias a P-Channel Mosfet without Exceeding Source-Gate Max Voltage

In Dave's answer, he refers to the transistor as being the current sink. Now my question is this:

Imagine, hypothetically speaking, I have a P-Channel mosfet IRF4905. And I drive a resistive load of -20 amps through the Mosfet. I am also driving the base of the transistor with a 10V signal. Would the current through the transistor be the 10V/1000R or would the current seen through the transistor be 20amps as well?

  • \$\begingroup\$ Can you please draw as schematic? \$\endgroup\$ – DKNguyen Mar 11 '19 at 0:34
  • \$\begingroup\$ @toor Ive added a snip of the original posts schematic with what I am trying to ask. I hope it makes sense now?? \$\endgroup\$ – Paul Ferreira Mar 11 '19 at 0:57
  • \$\begingroup\$ Q1 will pass I = (10V+0.7V)/1kohm = 10.7mA as long as everything above it still allows enough voltage to remain at Q1 collector. Q1 doesn't care what is happening above as long as their is enough collector voltage. Q1 is just being used to allow a ground referenced control voltage to control M1 since M1 is controlled by voltage between M1's gate pin and source pin (ie. since source pin is connected to 100V rail, M1 is controlled by a 100V rail reference signal). Can't directly control M1 with a ground referenced voltage because of this. \$\endgroup\$ – DKNguyen Mar 11 '19 at 4:31
  • \$\begingroup\$ If M1's gate-source could tolerate -100V being applied to it you would not need R2 and could turn on Q1 full blast and pull M1's gate straight to ground, but then you would need a base resistor to limit base current since R2 is also doing that right now). R2 provides negative feedback so Q1 doesn't turn on completely and pull M1's gate so low that it blows. \$\endgroup\$ – DKNguyen Mar 11 '19 at 4:35
  • \$\begingroup\$ Okay, apparently you meant current through the BJT, not the FET, so my answer is wrong and has been deleted. \$\endgroup\$ – Hearth Mar 11 '19 at 13:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.