What's the difference of OTA and OPAMP?

[frankiebai]

lm13600 opamp

Hi,
We can always see design and use of opamp in the textbooks.
and what's the difference of OTA and OPAMP?

 

[LvW]

voltage limit ota

You can imagine - as a rough approach - that an OTA is an opamp without the last stage.
1.) More correct: An OTA (operational transconductance amp) is an amplifying unit with a current output (high output resistance) in contrast to the classical opamp having a low output resistance (voltage source).
2.) Therefore, the transfer characteristic of the OTA is "Transconductance gm" (current divided by input voltage).
3.) Important feature: Commercial OTA´s have a separate pin to control the bias current for the internal diff. amplifier resp. the transconductance gm.
4.) An output voltage is created only if there is a load impedance (but often an additional buffer is needed)
5.) OTA allow negative feedback although normally it is used without feedback.

 

[coolstuff07]

commercial ota

1.) OPAMP: Differential high impedance input and very low impedance output (voltage source) and a very high gain.

2.) OTA: Differential high impedance input and rather high impedance output (current source).
Thus, the relation between output and input is current/voltage=transconductance gm which has a final value. As an important feature the value of gm in commercial OTAs can be varied by a current into a separate pin.

Application:

3.) Opamps always are used with external feedback in order to create amplifier, filter, adding circuits, integrators,...... (Of course, they can drive also capacitive loads up to a certain limit)

4.) OTAs are used normally without external feedback (although it is possible and allowed). They have better high frequnecy capability than opamps. The output voltage is simply output current times load impedance. Since gm can be varied the gain can be controlled by an external control voltage.

 

[LvW]

current differencing opamap

Hello coolstuff07,

it is a great honour for me that you have repeated a contribution from my side, dated 02.06.2008, 18:39.

Are there also some original information from your side ?

 

[xulfee]

ad ota amplifier

The LM13700 series consists of two current controlled operational transconductance amplifiers (OTA) each having differential inputs and a push-pull output. The LM13700 is like a standard op-amp: both have a pair of differential inputs and a single output, but an OTA is voltage in and current out rather than voltage in and voltage out; and OTAs are programmable via the IABC pin. Linearizing diodes at the input reduce distortion and allow increased input levels. The darlington outout buffers provided are specifically designed to complement the wide dynamic range of the OTA. This chip is very useful in audio electronics especially in analog synthesizer circuits like voltage controlled oscillators, voltage controlled filters, and voltage controlled amplifiers. The darlington output buffers on the LM13700 are different from those on the LM13600 in that their bias currents (and hence their output DC levels) are independent of IABC pin. This usually results in performance superior to that of the LM13600 in audio applications.

 

[suria3]

opamp with push pull output

Normally OTA is a one stage amplifier like a folded cascode architecture which only have one pole and OPAMP is a two stage amplifier which will have 2 poles. In terms of characteristic between 2 of them, you can refer to what others have explained. The book of Philip Allen explain well of this and the application.

 

[LvW]

opamp load impedance

Normally OTA is a one stage amplifier like a folded cascode architecture which only have one pole and OPAMP is a two stage amplifier which will have 2 poles. In terms of characteristic between 2 of them, you can refer to what others have explained. The book of Philip Allen explain well of this and the application.

That´s a bit too simple. Of course, the opamp contains more than only two stages.
And, of course, the opamp has more poles than two. Otherwise. we would never have a stability problem with feedback.