# PNP Differential Amplifier

Discussion in 'Homework Help' started by Darkcat, Apr 16, 2013.

1. ### Darkcat Thread Starter New Member

Apr 16, 2013
2
0
It has been quite a while since I have had to deal with BJTs and have no clue where to even start I have to use PNP transistors to create a single-ended inverting output, differential amplifier.

Collector Resistance = Load Resistance = 1.5K Ohms
Current Value = 4.5mA

This is about all I know about this project :\ Any help, even a basic understanding of how PNPs work, would be very much appreciated.

2. ### takao21203 AAC Fanatic!

Apr 28, 2012
3,785
502
Interestingly many NPN circuits can be changed to use PNP instead.

The gate or here base voltage is negative. Current flows out of the base.

3. ### crutschow Expert

Mar 14, 2008
20,531
5,817
Generally you can take an NPN circuit, substitute PNP's for the NPN's and reverse the polarity of all voltage sources.

Is there any gain or voltage output requirements for this amp?

4. ### WBahn Moderator

Mar 31, 2012
23,396
7,102
If you have "no clue where to even start" and need help with "even a basic understanding of how PNPs work", then it sounds like the place to start is by retaking the prerequisite course(s) for the one you are in. If this is truly an accurate statement of your level of preparedness, the proceeding on will just lead you further down a hole until you are trapped in a place where you just flounder around aimlessly. The sooner you address these difficiencies, the better.

5. ### Darkcat Thread Starter New Member

Apr 16, 2013
2
0
Perhaps "no clue where to start" is not an accurate statement. I can identify a PNP, as well as, the collector, emitter, & base tail. I remember the voltage difference required to make sure it is in ACTIVE mode. Creating a current mirror with NPNs is also something I recall. What escapes me is how to create a differential amplifier with PNPs. It is quite easy to forget certain things when not used often.

6. ### WBahn Moderator

Mar 31, 2012
23,396
7,102
Do you recall how to make a differential pair using NPNs? If so, work the problem as though that was what you were asked to do. Then worry about flipping everything later. You need to start from something you remember how to do and work your way forward.