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Design >> Analog Design >> Regulated Cascode
https://designers-guide.org/forum/YaBB.pl?num=1249120968

Message started by raja.cedt on Aug 1^st, 2009, 3:02am

Title: Regulated Cascode
Post by raja.cedt on Aug 1^st, 2009, 3:02am

hi,
can any body please tell me whats the main difference between these two circuits? what i see is manly in the first ckt we can keep vds of M1 constant with applied reference voltage..but in the second one by changing the currents we can change..i am expecting apart from this

Thanks,
rajasekhar.

Title: Re: Regulated Cascode
Post by jiesteve on Aug 2^nd, 2009, 8:56pm

Not sure what's going on with the second circuit. You can build a single transistor error amplifier to accomplish the same with the attached ckt.

T2 = Cascode
T3 = error amplifier

Title: Re: Regulated Cascode
Post by raja.cedt on Aug 2^nd, 2009, 9:49pm

hi,
i know the ckt what you have given.but what i want to ask is is output impedance is same in both case's what i have given?
i think ans is yes.

Thanks,
rajasekhar.

Title: Re: Regulated Cascode
Post by Berti on Aug 3^rd, 2009, 12:00am

I am not completely sure, but the circuit of Steve has the problem that T3 has to be operated in subthreshold region if vds of T1 is not sufficiently high.

T4 might circumvent the problem by providing a levelshift. However, the bias current through T4 must be small compared to the main branch.

Cheers

Title: Re: Regulated Cascode
Post by RobG on Aug 3^rd, 2009, 8:44am

raja.cedt wrote on Aug 2^nd, 2009, 9:49pm:

hi,
i know the ckt what you have given.but what i want to ask is is output impedance is same in both case's what i have given?
i think ans is yes.

Actually, the answer is maybe. The output impedance is boosted by the loop gain of the feedback network. For example, if you use a 2 stage opamp it will produce a higher output impedance than the circuit on the right.

rg

Title: Re: Regulated Cascode
Post by RobG on Aug 3^rd, 2009, 8:57am

Berti wrote on Aug 3^rd, 2009, 12:00am:

T4 might circumvent the problem by providing a levelshift. However, the bias current through T4 must be small compared to the main branch.

Cheers

Sorry for dominating these conversations, but I hope I'm being helpful.

The circuit on the right has been used for a long time. It is a wide swing regulated current source. As you guessed, M4 provides level shifting so that M1 has the minimal voltage across it without going linear. The current does not have to be small, but M1 must be sized to handle it. In fact, if the current is too small it will reduce the phase margin of the gain boosting circuit.

rg

Title: Re: Regulated Cascode
Post by raja.cedt on Aug 3^rd, 2009, 2:01pm

hi guys,
thanks for your answers.

@berthi: hi,i understand your reply..but is there any problem if it is in sub threshold region, what i am thinking is loop will be very slow and may effect stability..please correct me if any thing wrong..

@RobG: How low current will decrease PM?

Thanks,
Rajasekhar.

Title: Re: Regulated Cascode
Post by RobG on Aug 3^rd, 2009, 4:33pm

raja.cedt wrote on Aug 3^rd, 2009, 2:01pm:

@RobG: How low current will decrease PM?

M4, M3, and M2 create a local feedback loop. M4 just acts as a level shifter, but it's impedance (1/gm) will react with the gate-source cap of M3 and introduce a pole. Therefore you want the current through M4 to be large enough so that the pole doesn't affect the phase margin of this local feedback loop.

Title: Re: Regulated Cascode
Post by jiesteve on Aug 11^th, 2009, 1:48pm

In the second circuit, the level shifter (M4) could also be implemented as a source follower (instead of the diode connected device). Upside is that there is no error current going into the mirror... What would be the downside(s)?

Title: Re: Regulated Cascode
Post by RobG on Aug 11^th, 2009, 7:11pm

AnalogDE wrote on Aug 11^th, 2009, 1:48pm:

Unfortunately the source follower would need to be a PMOS its Vt would not track with the NMOS amp (M3) Vt. This is needed to provide the proper Vds for M1. The only way I've been able to do it without the diode connected device is to use a PMOS diff pair opamp.