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what's the transfer function of this ciucuit?

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walker5678

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what is transfer function

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transfer function

Hi

Please read a book like "Microelectronic Circuits" by Sedra & Smith

Regards
 

transfer function operational amplifier

in s-domain,

Vo/Vi (s) = -[1+sCR2] / R1R2

please correct me if i'm wrong..
 

find transfer function op amp

Hi

The correct answer is Vo/Vi (s) = -[R2/R1] * [1/(1+sCR2)].

If you have DC the gain should reduce to -R2/R1 and decrease as you go up in frequency.
 

two stage transfer function

oo.. yup.

i've made a mistake... thank you for the correction..
 

transfer function of resistance

I mean if the opamp is not ideal(for most cases, it is the truth), and it's gain is A, output resistance is Ro, what's the transfer function?

Found it's very complicated.
 

cmos transfer function sense amplifier

You'll find a derivation of closed loop transfer function in text books as Gray Meyer Analysis and Design... I'll remember, that I calculated similar circuits at times, also transfer functions of higher order active filters.

I admit, that I'm content in most cases with a qualitative analysis: How does the output resistance affect the transfer function? How does the transfer function modify at the GBW limit? In some cases, a simple pole-zero gain expression can be estimated.

In other cases, particularly when higher orders of the amplifier gain function must be considered, a simulation is necessary.
 

operational amplifier transfer function

walker5678 said:
I mean if the opamp is not ideal(for most cases, it is the truth), and it's gain is A, output resistance is Ro, what's the transfer function?
Found it's very complicated.

Do you really want to include the output resistor ? Why not also the input impedance ?
I propose only to consider the finite gain A, since the contribution of Ro is really negligible.

Then, the transfer function is

G=AHf/(1+AHr) with the return function Hr=R1/(R1+Xp) and Xp=R2||(1/jwC)
and the forward function Hf=Xp/(R1+Xp)
 

simple ota transfer function

Do you really want to include the output resistor ? Why not also the input impedance ?

The ideal opamp's output resistance is zero, while actural opamp's ouput resistance is very large, for example, several million ohms, especially for the OTA. And if you want to minimize the output resistance, you must increase either the output transitor size or the output stage bias current, or use the output buffer stage. All these ways will introduce bargain with the die size, power consumption, and circuit complication , and sometimes may be not applicable.
On the other hand, the input resistance is normally very large for CMOS process due to the gate dielectric. So it can be ignored.
 

walker5678 said:
The ideal opamp's output resistance is zero, while actural opamp's ouput resistance is very large, for example, several million ohms, especially for the OTA.
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Correction: The "actual opamps output resistance" is rather low. The figure from walker5678 shows an opamp and NOT an OTA. An OTA is no opamp but a transconductance amp. These terms should not be mixed in order to avoid misunderstandings.
 

The "actual opamps output resistance" is rather low.

Why?

For the simple two stage amplifier, if the Ids of output stage is 1mA(which is very large for CMOS IC), and lambda is 0.01, then the output resistance is about 0.5/lambda*Ids = 50K.
So is the simple two stage amplifier an operational amplifier or an OTA?

OK, so most of the onchip amplifier is essentially OTA, which has large output impedence, thus if you want to add a pole or zero in the control loop, maybe it's hard to estimate the precise location of these poles/zeros, ... unless use the simulation method.
 

walker5678 said:
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For the simple two stage amplifier, if the Ids of output stage is 1mA(which is very large for CMOS IC), and lambda is 0.01, then the output resistance is about 0.5/lambda*Ids = 50K.
So is the simple two stage amplifier an operational amplifier or an OTA?
Neither - nor.
But, of course, it´s a matter of definition.
I would call your two-stage amplifier simply "amplifier".
The term opamp applies - for my opinion (and so it is used worldwide) - to amplifiers which have a very high gain and input impedance and a very low output impedance.
Only in this case you can create mathematical "OPERATIONS" which are determined solely by the feedback network.

Conversely, an OTA is an amplifier which can be seen as a voltage controlled current source. But it is clear that this is true only for a limited range of load impedances.
 

hehe.... I know your meaning. It's really a difference between naming conventions.

We call the two stage amplifier i mentioned the "non-ideal opamp" , and the opamp you said the "ideal opamp".

Any way, the discussion makes it more clear for the difference between opamps and OTA.
 

walker5678 said:
hehe.... I know your meaning. It's really a difference between naming conventions.

We call the two stage amplifier i mentioned the "non-ideal opamp" , and the opamp you said the "ideal opamp".

Any way, the discussion makes it more clear for the difference between opamps and OTA.

OK, I see. It´s really necessary to find an agreement on terms and conventions.
For me, an ideal opamp has really "ideal" parameters: A infinite (or sometimes 1E6), rin infinite, rout=0.
And the "real" opamp is something between ideal and real:
A finite and frequency dependent; rin and rout are considered as far as it makes sense.
 

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