What is the usage of the PMOS in this simple amplifier?

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Davidy

Joined: 14 Sep 2005
Posts: 20

29 Oct 2006 13:00 What is the usage of the PMOS in this simple amplifier?

This is a simple amplifier in a typical bandgap circuit.
What is the usage of the PMOS in this simple amplifier?

Who can tell me why there is a PMOS FET "ST1" in the middle of the circuit?
what is the useage of it?
Thanks!

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DZC

Joined: 12 Sep 2006
Posts: 158
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29 Oct 2006 13:37 What is the usage of the PMOS in this simple amplifier?

Maybe it acts as a start circuit to get rid the dead zone of the differencial pair.

isaacnewton

Joined: 13 May 2004
Posts: 158
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Location: Earth Village

29 Oct 2006 23:13 Re: What is the usage of the PMOS in this simple amplifier?

There is no dead zone for differential pair.

DZC wrote:

Maybe it acts as a start circuit to get rid the dead zone of the differencial pair.

Fom

Joined: 10 Mar 2004
Posts: 778
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Location: Taiwan

30 Oct 2006 8:59 Re: What is the usage of the PMOS in this simple amplifier?

I think this PMOS for monitoring VDD. If this bandgap is used for internal LDO for VDD it help to startup this LDO+bandgap combination.
If VDD still low it will speed up switching it ON.

Davidy

Joined: 14 Sep 2005
Posts: 20

30 Oct 2006 11:49 Re: What is the usage of the PMOS in this simple amplifier?

Fom wrote:

I think this PMOS for monitoring VDD. If this bandgap is used for internal LDO for VDD it help to startup this LDO+bandgap combination.
If VDD still low it will speed up switching it ON.

I can not master your meaning. When will that PMOS on? How it can speed up switching?

sengyee88

Joined: 09 Oct 2006
Posts: 124
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30 Oct 2006 17:14 Re: What is the usage of the PMOS in this simple amplifier?

Without STI transistor, When start-up, ie VDD ramp up from 0V to Vcc, internal nodes can be any voltage. If it happens that drain of P49 transistor is at Vcc, no current will flow through the differential pair as Vsd=0, and the differential pair remains "dead" even if supply is on.

With proper sizing of STI transistor, it ensures that P49 is always working in saturation region during start up, Vsd=Vsg_P49-Vsg_STI>Vsg_P49 - |Vth_P49|. This will trigger the differential pair to work as normal upon supply ramped up.

mists

Joined: 14 May 2002
Posts: 380
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Location: China

31 Oct 2006 4:12 Re: What is the usage of the PMOS in this simple amplifier?

sengyee88 wrote:

but if the input of the opamp is normal(low than vdd-vsd(p49)-vtp), it will pull down the drain voltage of p49, then it leave "dead" and work normal.

sengyee88

Joined: 09 Oct 2006
Posts: 124
Helped: 20

31 Oct 2006 10:52 Re: What is the usage of the PMOS in this simple amplifier?

Quote:

but if the input of the opamp is normal(low than vdd-vsd(p49)-vtp), it will pull down the drain voltage of p49, then it leave "dead" and work normal.

How you ensure this "if" condition always happens? This is a start-up circuit, it has to ensure itself that it works as normal in any condition when power is up.

wpchan05

Joined: 16 Feb 2006
Posts: 165
Helped: 11

31 Oct 2006 11:24 Re: What is the usage of the PMOS in this simple amplifier?

sengyee88 wrote:

Quote:

but if the input of the opamp is normal(low than vdd-vsd(p49)-vtp), it will pull down the drain voltage of p49, then it leave "dead" and work normal.

How you ensure this "if" condition always happens? This is a start-up circuit, it has to ensure itself that it works as normal in any condition when power is up.

But will this PMOS switch off after it starts up the differential pair?

sengyee88

Joined: 09 Oct 2006
Posts: 124
Helped: 20

31 Oct 2006 17:02 Re: What is the usage of the PMOS in this simple amplifier?

I guess it will not turn off. Having large L and small W can limit the current flowing through it,

wpchan05

Joined: 16 Feb 2006
Posts: 165
Helped: 11

31 Oct 2006 22:13 Re: What is the usage of the PMOS in this simple amplifier?

sengyee88 wrote:

I guess it will not turn off. Having large L and small W can limit the current flowing through it,

So, will this design be actually not a good one? I mean, there will be offset if we use the same aspect ratio for the differential pair.

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

01 Nov 2006 1:36 Re: What is the usage of the PMOS in this simple amplifier?

Under normal condition, the PMOS ST1 will not work :
The highest voltage of ST1's source terminal , and the drain of the P49 , will be
Vsst1 = VCC - Vovp49
But Vovp49 =VCC -Vbias - Vtp
So Vsst1= Vbias + Vtp , then the over drive voltage of ST1 should be :
Vovst1 = (Vbias + Vtp) - Vbias - Vtp = 0