Could this circuit be used?

A circuit which is consisted of a PMOS and a NMOS connected together (please see the attachment)

This circuit seems a little strange to me, because I've never seen such kind of circuit. I am wondering if such combination of PMOS and NMOS transistors can be used ? If not, are there negative effect which can be induced by this circuit?

the gate of PMOS is floating here...

This circuit with a little modification could be found in a gain boosted amplifier with PMOS load. Here the NMOS acts as the input of the gain boosting amplifer and PMOS forming part of the load of the main amp.

It's a positive feedback loop.

why is it a positive feedback loop?
Normally the positive feedback should be avoided in the circuit design, shouldn't it?----It seems like this circuit is not a good design

thanks very much

walker5678 said:

It's a positive feedback loop.

Click to expand...

yeah, it's a positive feedback. it could be used as a part of the latch.

Octago said:

why is it a positive feedback loop?
Normally the positive feedback should be avoided in the circuit design, shouldn't it?

walker5678 said:

It's a positive feedback loop.

Click to expand...

Click to expand...

You need positive feedback for tons of applications, like oscillators, monostable pulse generators, high gain comparators, etc.

This circuit could be part of any of the mentioned aboved (well... maybe not in oscillators, but you never know).

Remember that "bad designs" don't exist!!! All circuit configurations can be used for different types of things, it all depends on the application!!

Hope this helps,

diemilio

thanks a lot!

But I still have not understood why is this circuit a positive feedback. Could someone explain it?

Do you have some reference document(for example. paper) of oscillator or monostable pulse generator based on this circuit?

thanks

diemilio said:

Octago said:

why is it a positive feedback loop?
Normally the positive feedback should be avoided in the circuit design, shouldn't it?

walker5678 said:

It's a positive feedback loop.

Click to expand...

Click to expand...

You need positive feedback for tons of applications, like oscillators, monostable pulse generators, high gain comparators, etc.

This circuit could be part of any of the mentioned aboved (well... maybe not in oscillators, but you never know).

Remember that "bad designs" don't exist!!! All circuit configurations can be used for different types of things, it all depends on the application!!

Hope this helps,

diemilio

Click to expand...

Well this circuit could be used to increase the output resistance in a current mirror....

well guys to add a twist to the tale....the combination seems more like a SCR....i think the operation would resemble more like that...but only way to find it out is to implement the circuit....

jaywant007 said:

well guys to add a twist to the tale....the combination seems more like a SCR....i think the operation would resemble more like that...but only way to find it out is to implement the circuit....

Click to expand...

mmm... I don't think so... SCRs are modeled just as this circuit but using BJTs because to activate the positive feedback you need current through one of the transistor's base. MOSFETs can't have current through their gate, so this won't be a proper model for an SCR.

Added after 15 minutes:

Octago said:

Do you have some reference document(for example. paper) of oscillator or monostable pulse generator based on this circuit?

thanks

Click to expand...

Well... here's an application. It's a positive feedback comparator.

**broken link removed**

Notice how the collector of Q7 is connected to the base of Q8, and then the collector of Q8 goes to the base of Q7 forming the positive feedback loop. It's basically the same concept used in your schematic, but with NPN BJTs. As you can see the circuit consists of an amplifier and a latch. A latch could be used as a monostable or bistable depending in the way you connect it's terminals.

Hope this helps,

diemilio