Comparison of two stage opamp and folded cascode opamp.

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legend

Joined: 17 Jan 2004
Posts: 20

21 Jan 2005 6:17 folded cascode op amp design tutorial

Guys! who can tell me advantage and drawback of two -stage opamp and folded cascode opamp?

Best regards!

wee_liang

Joined: 04 Jan 2005
Posts: 26

21 Jan 2005 6:30 two stage folded cascode

two stage opamp
- requires internal compensation

folded-cascode opamp
- require more voltage biasing
- requires more vdd headroom
- noisier
+ compensation through external load C

That's what I recall from the top of my head ..

Mline7

Joined: 28 Dec 2004
Posts: 34
Helped: 1
Location: Marseille, FRANCE

21 Jan 2005 15:45 resistive load differential pair op amp

It seems to me a bit strange to compare a two stage architecture to a single stage architecture.

Generally talking, single stages offer lower gain, limited output swing and external stabilization compared to two stages architectures.

As for the folded cascode architecture also offers some advantages:
- High Gain
- High BandWith
- Limited output swing
- Low Voltage Functionnality

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rfsystem

Joined: 25 Feb 2002
Posts: 858
Helped: 96

21 Jan 2005 21:45 op amp comparison cascode two stage telescopic

All are wrong!

The folded cascode is a technique to extend the common mode input range capability to one rail or a little beyond. Also the voltage driving capability for a two stage design or the output voltage range for a one stage design is extended.

One stage designs are only used if you have only small cap loads. The drwaback of the folded cascode is that folding node creates a parasitic pole. So maximum gain bandwidth product is limited independend from the minimum gain for which the opamp should be stable.

jiangwp

Joined: 27 Jul 2004
Posts: 184
Helped: 9

22 Jan 2005 5:28 folded cascode bandwidth

Generally, the folded-cascode is single stage amplifier to apply for most applications. The drawback of it is that the output swing of it is small due to the stack of cascode transistor.

If a large drive capability is need, a buffer stage is added.
If a large swing of output is need, another stage is added to extend the swing.

cetc1525

Joined: 08 Oct 2004
Posts: 177
Helped: 3

23 Jan 2005 5:20 folded cascode operational amplifier architecture

i think folded-cascode can provide more gain.but need more voltage supply.

rfsystem

Joined: 25 Feb 2002
Posts: 858
Helped: 96

23 Jan 2005 23:15 operational amplifier architecture folded cascode

Again: The folded cascode does not relate to single or double stage design. The folded cascode also extend the output driving capability to the MOS input of the output stage. Otherwise input common mode range and output stage driven is in conflict. The folded cascode could be applied for single and double stages. A single stage opamp is a gm-stage, a double stage opamap has an intermediate node and have to be compensated for unity gain stability.

Mline7

Joined: 28 Dec 2004
Posts: 34
Helped: 1
Location: Marseille, FRANCE

24 Jan 2005 13:45 advantages folded cascode operational amplifier

The Folded Cascode Architecture allows to put a cascode differential pair in a low supply voltage environnement, thereby allowing to also cascode the output load, and so to obtain high gain stage, still in a low voltage environnement.

However, this architecture, as every cascoded architecture suffers from limited output swing compared to their non-cascoded counterparts.

The foded-cascode architecture also have the advantage of reducing the input capacitance of the circuit. In fact, while in non-cascoded differential pair structure, the cgd capacitance is multiplied by the amplifier gain when seen at the input, it is only multiplied by the casode gain in cascoded differential pair (approximately gm_diff/[gm_casc+gmb_casc] ). So, it lowers the previous stage's load, or reduce the output load in feedback, thus providing higher bandwith than the non-cascoded counterpart.

cliffj

Joined: 22 Mar 2004
Posts: 57
Helped: 1

14 Oct 2005 1:18 two stage opamp architecture

Better ICMR in folded cascode. Better open loop gain in two stage opamp design.

analoger

Joined: 29 Sep 2005
Posts: 23
Helped: 1

14 Oct 2005 1:37 two-stage folded cascode operational amplifier

Two stage opamp has lower bandwidth,poor PSRR due to the compensation capacitor.

Alles Gute

Joined: 04 Dec 2003
Posts: 142
Helped: 5

14 Oct 2005 14:35 Re: Comparison of two stage opamp and folded cascode opamp.

analoger wrote:

Two stage opamp has lower bandwidth,poor PSRR due to the compensation capacitor.

It should be: poor PSRR due to the compensation capacitor in high frequency.

Added after 10 minutes:

There are many topologies exists in the op amp design world. Cascode topology, including telescopic cascode, folded cascode and regulated cascode, is one of the most important. Each of them has their own plus and minus. Which topology is the best choice depends on the application especially depends on the voltage supply. At large supply voltages, the telescopic architecture becomes the natural choice for systems requiring moderate gain from the op-amp. Reducing supply voltages, on the other hand, forces reconsideration in favor of the folded cascode, or, in the extreme case, the two-stage design.
There are a lot of literature resources you can refer to for this problem. For example:
PAUL R. GRAY, “Mos Operational Amplifier Design: A Tutorial Overview”IEEE J. Solid State Circuit, Vol. SC-17, No. 6, December 1982

nani

Joined: 17 Mar 2005
Posts: 29
Helped: 1

24 Oct 2005 23:17 Re: Comparison of two stage opamp and folded cascode opamp.

folded cascode

advantages
high gain, no need for compensation, good psrr

disadv

need CMFB circuit, bais ckt, high o/p impedance,o/p swing is low, common mode range either on poitive or negative side will be limited u haveto go for fully complementary to have good cmr

two stage

advantages

good o/p swing, b.w will be high compared to folded[by using nulling resistor] , good gain,low noise compared to folded

disadv

need compensation, poor psrr

skal81

Joined: 19 Jan 2005
Posts: 93
Helped: 16

25 Oct 2005 0:03 Re: Comparison of two stage opamp and folded cascode opamp.

I don't know what you guys think about this, but to compare the amp we also need to compare there application.

I mean that cascode amp is generally used to drive a capacitive load, so used on switched cap for eg.
But 2 stage amp can drive a resistive load if the second stage is a low impedance buffer.
What do you think ?