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296 Threads found on edaboard.com: Opamp Output Impedance
I am trying to measure the output impedance of an opamp in the attached configuration. Results are attached. Basically, Zo = Vo/Io = 10mV/40mA = 0.25 Ohms. Without the capacitor in the circuit, I get mostly a large dc current. Is there a better method to measure output impedance? I would like to (...)
what's the output impendence do you preferring? a R-R opamp plus push-pull MOSFET pair is an option. mike -------------------------------------------
hi To get a low output impedence (need when drive resistive load), you can use opamp as a buffer, prefer use rail to rail opamp because r2r output can go to very low voltage. If your output driving capacitive load, high output impedence has no problem to drive, for example gate of (...)
Are you doing this with an actual device or in simulation. Either way if you disconnect all the supplies and then calculate the output impedance you will definitely get such a high resistance since all the MOS or BJT are cut off. output impedance in this case means the small signal output (...)
How to simulate the output impedance? Thanks!
Here Measuring output impedance of the opamp with closed loop
Thanks supposing your amplifier has a one dominant pole, you can plot the Bode's diagram of the opamp gain and then calculate: Rout=Aol/Gm where Aol is the DC gain you can read from the Bode's plot and Gm is the global transconductance of the opamp (which value also depends on with how many stages the opamp is built
XF is very usefull for CMRR and PSRR characterization. output impedance u are able to easy obtain using AC analysis. Just put AC voltage source (e.g. 1V) with 0V DC in series with output of amplifier. Measure AC voltage at ouput and Ac current flow though output: Zout=Vout/Iout P.S. Generally when you simulate (...)
Hello all, Would some one help me to figure out the formula of the output impedance of a class-AB opamp? I attached the schematic of the opamp. My purpose is to reduce the output impedance of the opamp to about 5 KOhm (currently, it is 28 KOhm).
... It should deliver as much power as possible to a 20 ohm load. Therefore, I am trying to make the output impedance as near to 20 ohm as possible. No, to achieve this, Rout ≪ 20Ω ! Thus, obviously I should use a common drain MOSFET. However, the problem is output impedance of a common drain mos
If you neeed output resistance to be exactly 50Ω, I would recomend you to use an opamp voltage follover and attach a 50Ω resistor to it. With your circuit, you have to know the output resistance before you can alter it by adding a resistor to it. I'm sorry to say I can't calculate the current output (...)
Why is that the output impedance of the OTA is ideally very high? I thought we need a high output current but why is it that the ideal output impedance is very high? Can you please explain it to me? In contrast to the well known opamp (which is a voltage source) the OTA shall act as a (...)
Hi dino, two additional remarks: What do you mean * with "ideal amp"? Transistor, opamp, OTA? * with Rout? (The term Rout in your formula primarily consists of the load resistance and - in most cases - is not the output resistance of the amplifier itself).
Hi all, what we have learnt is that the output impedance of an opamp is very small. I cannot seem the find this information in the datasheet of the opamp I am looking at. May I know how can I measure the output impedance of an opamp using only spice?
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
It all depend on the specification that you are looking for. Most of the time the process we use to design a opamp will be the crucial point as it will limit the design parameters and the rest as others mentioned in the post.
opamp is the amplifier that has the characteristic close to the ideal amplifier. The ideal amplifier has infinite gain, infinite input impedance and zero output impedance.
Hi, You could do it this way - add an opamp to buffer the output voltage from the digipot. How much current do you need from the output? If you need more than the opamp can give, use a power opamp or add an emmiter-follower with a transistor after the opamp. Any reason you don't want (...)
Due the application of dc feedback, I think a typical differential stage opamp would serve the purpose. If the frequency of application is high, a cascode stage differential opamp would serve the purpose Rgds
high impedance at input is needed so as to pass whole of the upcoming voltage to the input of opamp. low impedance at o/p is needed so as to pass whole of the o/p voltage to next stage.
Importantly, is the Y-axis right? I mean you have a Y axis which calls of degrees. I do not think so. I do not think that there is an opamp which will cause a 5 degree phase shift. Secondly, have you put the options as .OPTIONS UNWRAP for the phase plot? If you have done so, then the cause is due to two RHP zeroes and not multiple poles. The pr
Well, both can be explained using voltage divider rule. Assuming the opamp input is connected to a voltage source having a source resistance of Rs and the opamp input impedance is Ri, the voltage going into the opamp is Ri/(Ri+Rs). As can be seen, we would like to have Ri as large as possible. The low output (...)
hi folks, Can suggest any achitecture available to desgin diff. to single ended opamp using npn & resistor only ... pnp /pmos not availabe in thise process. thanks in advance
How can i get an expression to the output resistance of the super cascode opamp shown? any help. Thanks
I have tried this method, I think it's right. You can reference: 1. Connecting your opamp to buffer structure. Run .TF, you can find the output impedance. It's very small, because it's close loop output impedance. 2. Changing structure to run AC simulation. You can get low frequency gain. (...)
Hi opamps usually have output stage (Class A or Class AB) in order to have low output impedance. Even if an opamp doesn?t have an output stage, since it has a high gain and it is always used in a feedback loop, the output impedance of the (...)
One significant implementation could be use with a CMOS current mirror opamp with the output stage being a regulated cascode current mirror. Gains of about 100 dB have been achieved with this configuration.
In conventional two stage opamp the miller cap is added between first dtage and second stage. This is frequency compensation and helps in making one pole as dominant pole and pushing the other pole towards infinity. For the second pole ater compensation the RC product is approximately going to be same so,
It depends on your buffer style. If you buffer opamp as OTA, then you need two caps--one between the first and the second stage, the other between the second and the third stage. If you use souce follower, then you only need one cap between 1st and 2nd stage.
Some opamps are stable when driving a large capacitive load. Check the data sheet - this feature is usually highlighted on the first page.
why is it said that opamp got infinite input impedance?
Hi all, Please refer the following diagram of two stage opamp..Vdd = 1.8V, Vbias = 1.234V, Cload = When I give 900mV as input to both input terminals the output is 900mV and all the transistors are in s
transistor is single stage amplifier where as opamp is two stage amplifier. it can be 3 or more depend on the spec
can u state your question little more clearly.... what are you referring here as output point of an opamp.....
please tell me how the common mode votage is controlled in the opamp? thanks
Can any explain in details what is the difference between OTA and opamp.
1.Design an operational amplifier circuit for a signal conditioning application,where the signal of a microphone is conditioned for an AD-converter. -Microphone signal peak-to-peak value is 122mv. -Microphone signal has zero-bias, i.e it varies equally on both sides of zero -Microphone output impedance is 600 Ohms -AD-converter has 0 V to 2.5 V
The lousy old 741 opamp has poor performance because it is 40 years old this year. The DC and very low frequency gain is about 200,000 so it is extremely difficult to measure. It will amplify its own noise which is pretty high. It will amplify its own input offset voltage which could be up to 5mV. You measure the input and output (...)
I need to design 250Hz sallen key filter and looking for good opamp. I only have single supply at 3volts. I used AD824 and my design works for +3volts and -3volts supply. But it doesnt work for +3volts and 0 supply although the data sheet says that its a single supply opamp. I cant push the input to 1.5votls as I need to use several stages and a
I want to design an opamp (using in negative feedback like a buffer)which will drive transmission line of 10 meters,with impedance 50 ohms. I think ,If the transmission line is end terminated, a real 50 ohm resistor at the end. It means I will design the amp to drive a 50 ohms load in AC and DC , it is difficult. If the transmission line is
gain-boosting is good method to design high gain opamp. in my opinion, however, to design opamp which has 85dB gain use gain-boosting method is not appropriate. maybe you need optimization process for high gain opamp. the increase of bias current or output impedance to make opamp (...)
Hi abbeyromy Only now I have seen that your circuit has an error. The ground connection is wrong. Connect R1 as well as C1 to ground (instead of the + terminal) - and you have a good oscillator. However for frequencies in the kHz range don`t use the old 741 opamp because of its lousy slew rate. For R1=110k and R2=10k you get a rising
Could someone please explain the difference between an OTA and an opamp??? Is an opamp made of an OTA and a buffer stage? yes
Try to use opamp with unity gain configuration
Hi there, I am designing a two stage opamp (drives a capacitive load) and looking for a way to compensate it. The first stage is a typical pmos input and the second is a cascode one, please see the picture below (only shows the second stage). The "vb1, vb2 and vb3" are bias voltages generated somewhere else in the circuit. I am using the low he
Hi,i have designed a fully differential opamp.Let's say that my unity gain freq is x, after the open-loop simulations.When i close the loop with resistive feedback elements (R=100K at the feedback loop,R=10K at the input -- so closed loop gain to be 20dB) and i run ac analysis i notice at the bode diagram of the gain that the unity gain freq is muc
As the opamp has no negative feedback, I think its real output resistance Rout has to be taken into account. Together with the RC parallel load this forms a classical rc low pass - however, the corner frequency depends on Rout, which is not a fixed parameter. But perhaps, accuracy is not a main issue?
I have a fliter that uses opamp based integrator. I want to make the same filter but using OTA-C instead of opamps. But I'm confused on the process of doing that. Anyone can help me on how to get the same behavior from the transition from one to an other?In other words translating the RC values in the omAmp version into the OTA version with the sam
Hi, Thank you for watching this post. For a simple feedback, like R1=R2=R, opamp's gain is A. Non-inverting feedback amplifier, the input impedance is (1+AB)*Ri, B is feedback factor. Inverting feed amplifier , the input impedance is R+R/(1+A) I can do some math to get the result. But how to intuitive analysis (...)
Is there a somewhat simple way to determine if an opamp load is capacitive? How do you know, exactly? I am trying to use an opamp as a simple unity gain source follower, but all the datasheets have huge sections on special requirements for capacitive loads. I can not control the load of the opamp, it is a pre-existing circuit, and not (...)