The Output Resistance of an op amp

[Edward_2288]

output resistance of op amp

Hi,

what does it mean by the output resistance of an op amp? How do we calculate it?

Thanks.

 

[flatulent]

how to calculate the output resistance of op amp

The thevenin equivalent circuit of an amplifier is a voltage source in series with a resistance. To calculate it you need the schematic of the amplifier. The output stage is usually an emitter follower or a source follower. For the source follower it is 1/gm and for the emitter follower it is (1/gm + the driving impedance/beta)

 

[maxwellequ]

emitter follower op amp

In addition to the explanation already given, note that the output resistance is a parameter that usually varies with the load and output voltage (the gm of the output transistors depend on these parameters).

Note that feedback reduces the output resistance of an opamp.

Regards

 

[Edward_2288]

how to measure output resistance of an op amp

Hi, thanks for your fast reply.

well, let us consider one stage differential input folded cascode op amp. am i right to say that the output impedance is the total impedance at the output terminal (including CL the load capacitor)?

 

[maxwellequ]

op amps driving resistive loads

Edward,

There is a difference between an opamp (which ideally has Rout=0) and a transconductance amplifier (which ideally has Rout=infinity). You are referring to the transconductance amplifier. So, forget what was said about the output impedance being the 1/gm of the source followers of the output....

Usually transconductance amplifiers are used with capacitive loads (which seams to be the case...) and what usually really matters is the output resistance, because it defines the gain (along with the gm of the input differential pair). Sometimes the output resistance is named output impedance. So probably what you want is the output resistance, whose expression can be found in any textbook.

The load is never part of the output resistance of an amplifier.

 

[Edward_2288]

calculating output resistance of an op-amp

hi maxwellequ,

got confused a bit with your explanation. i'm so sorry since I am a new starter.

well, about the op amp and transconductance amplifier. you said they are both different, i think you refer op amp as the voltage amplifier, while transconductances as OTA. am I right?

if this is the case, yeah . you are rite that they are different, in term of input and output.

despite the problem, so in general the transconductance amplifier has a large output resistance than the voltage amplifier, since the output impedance itself determines the gain. Am I right? furthermore, if it drives only capacitive loads, it needs not any buffer stage. am i rite to said that?

thanks.

 

[maxwellequ]

transconductance gm calculation of ota op amps

Yes, that's it.

Regards

 

[Edward_2288]

how to measure the output resistance of a op-amp

Hi maxwellequ

well, am i correct if i said that the folded cascode topology is one of the ota topology?

thanks.

 

[maxwellequ]

output resistance of amplifier

Let me see if I can explain this clearly to you ...

The ideal operational amplifier has infinite input resistance (no input current) and zero output resistance (no matter what load it has it is always capable of driving it). Real opamps can have a very large input resistance (can be considered infinite for most applications), if MOS transistors are used in the inputs. The output resistance is usually on the order of tens of ohms, and models the limitation of the amplifier in driving low resistive loads.

The OTA is different building block: it generates an output current proportional to the input voltage (it is a transconductor...).

Ideally it has infinite input and output resistance. Practical output resistances are 100k-10MEG.

Although the output "variable" of an OTA is the current, it can be used as a voltage amplifier when the loads have a very large resistance. This is because the gain is A=gm(RL//rout) where gm is the transconductance. If a small RL is used the gain will be small.

For this reason the OTAs used as voltage amplifier in switched capacitor circuits, when there is no resistive load limiting the gain.

Thus, to answer directly to your question the folded cascode is an OTA, because it has very large input and output resistances and generates an output current proportionally to the input voltage.

Finally, note that an opamp is usually an OTA followed by a source-follower based output stage.

Regards

 

[Edward_2288]

calculate output resistance of an opamp

hi maxwellequ,

I really do appreciate your explanation. thanks a lot. it is very clear and precise. thanks.

Edward

 

[layes]

output resistance of an op amp

small signal
gm Rout=gain

 

[andy2000a]

resistance of an op amp

many analog design need calculate Impedance out .. like USB PHY ..
but someone use "small R" shunt to Gnd and measure ,

another way use DC sweep IO pad

..

have anyone can tell me how to really simulation Output impedance
by hspice (in .tran)

 

[yyliang]

input resistance ota

for capacitator load,the output resistor can be very large,but for resistor load,the output resistor should be very small.

 

[kyrandia]

output resistance of practical opamp

I think one method can be used to get output res of opamp. Apply ac to the input, measure the ac output voltage and current . Then v/i should be the small signal output res, is it ?

As to USB's output res, I think it is easier to measure.
1.Make usb drive se1, apply different voltage to the ouput. example: v1 and v2. measure the output current. I1 and I2 . Then you can get output resistor. deltav / deltaI should be that.
2.Make usb drive se0, apply voltage to the output and measure the current. v/i