a basic question about application of opamp

[calven303]

a basic amplify circuit as follows.
the needed gain is 10.the power voltage is 1.8v.when the input dc voltage is 0.9v,because the feedback factor is 1/9,the output dc voltage should be 9v,so the opamp can not work.
how to resolve the problem?are there any other architechture?please introduce some![/img]

Added after 2 minutes:

the circuit schematic

 

[LvW]

a basic amplify circuit as follows.
the needed gain is 10.the power voltage is 1.8v.when the input dc voltage is 0.9v,because the feedback factor is 1/9,the output dc voltage should be 9v,so the opamp can not work.
how to resolve the problem?are there any other architechture?please introduce some!

There is no solution because of contradictory requirements.
It has nothing to do with opamp "architecture".
With a 1.8 power supply and a gain of 10 you cannot accept an input of 0.9volts.
Increase the supply voltage or decrease the gain or lower the input voltage.

 

[calven303]

the dc voltage of the input signal is 0.9,which is needed to bias the input nmos of the opamp.while the small signal maybe is sevral mv.even in this condition,there is no other solution?
the dc voltage is amplified together?
thanks a lot
regards

a basic amplify circuit as follows.
the needed gain is 10.the power voltage is 1.8v.when the input dc voltage is 0.9v,because the feedback factor is 1/9,the output dc voltage should be 9v,so the opamp can not work.
how to resolve the problem?are there any other architechture?please introduce some!

There is no solution because of contradictory requirements.
It has nothing to do with opamp "architecture".
With a 1.8 power supply and a gain of 10 you cannot accept an input of 0.9volts.
Increase the supply voltage or decrease the gain or lower the input voltage.

 

[LvW]

the dc voltage of the input signal is 0.9,which is needed to bias the input nmos of the opamp.while the small signal maybe is sevral mv.even in this condition,there is no other solution?
the dc voltage is amplified together?

OK, I summarize. Correct me if I am wrong:
Single supply 1.8 volts, ac input some millivolts, gain of +10.

Suggestion for the feedback voltage divider:R2/R1=9kohms/1 kohms.
But it is important that there is a capacitor between R1 and ground.
More than that, you need a voltage divider 1:1 in front of the +input to bias the opamp with 0.9 volts. And therefore, a second capacitor between ac source and opamp input.
It´s a classical amplifier arrangement for single supply.

 

[calven303]

thank LvW a lot!and what you just said is corret.but why the circuit can not work,and how to resolve the problem?
regards

 

[r2d2droids]

can u give the detailed schematic?

 

[dipnirvana]

bcoz' opamp will saturate

 

[calven303]

the dc voltage of v2 is 0.9v.the OTA is a one stage folded-cascode opamp.doing dc analysis,the circuit can not work.
regards

can u give the detailed schematic?

 

[LvW]

the dc voltage of v2 is o.9v.the OTA is a one stage folded-cascode opamp.doing dc analysis,the circuit can not work.
regards

Why didn´t you follow my advice to place a cap between R2 and ground ?
It´s absolutely necessary.
Question: Your sentence "...the OTA is a ......opamp" is misleading.
Do you work with an OTA or with an opamp ?

 

[calven303]

sorry,it is an OTA.i will try to add the cap,but the cap can change the dc characteristic?
regards

 

[LvW]

sorry,it is an OTA.i will try to add the cap,but the cap can change the dc characteristic?
regards

Both of the circuits provided by you up to now show an operational amplifier.
If you now plan to change to an OTA, neither the amplifier symbol nor the feedback network apply anymore.
Do you know the function and the principle of an OTA ?
Do you still want feedback ?

 

[calven303]

it is an OTA,although the symbol is labled OPA.because it is an OTA,the feedback network needs to be changed?and how?
regards!

sorry,it is an OTA.i will try to add the cap,but the cap can change the dc characteristic?
regards

Both of the circuits provided by you up to now show an operational amplifier.
If you now plan to change to an OTA, neither the amplifier symbol nor the feedback network apply anymore.
Do you know the function and the principle of an OTA ?
Do you still want feedback ?

 

[r2d2droids]

it is an OPA labelled OTA but doesnt work.
its very complicated.
i am confused...

 

[LvW]

it is an OTA,although the symbol is labled OPA.because it is an OTA,the feedback network needs to be changed?and how?
regards!

My last question was if you really want to have feedback !
Before designing or changing a feedback network you have to decide if there must be feedback at all ! (Remeber: OTAs do not need feedback !).

 

[calven303]

it is OTA labelled OPA!

it is an OPA labelled OTA but doesnt work.
its very complicated.
i am confused...

Added after 3 minutes:

i think i need feedback to get precise gain."OTAs do not need feedback !",can u explain it more?thanks a lot!
regards

it is an OTA,although the symbol is labled OPA.because it is an OTA,the feedback network needs to be changed?and how?
regards!

My last question was if you really want to have feedback !
Before designing or changing a feedback network you have to decide if there must be feedback at all ! (Remeber: OTAs do not need feedback !).

 

[LvW]

Quote:
i think i need feedback to get precise gain."OTAs do not need feedback !",can u explain it more?thanks a lot!
regards

Normally, OTAs are used without feedback. The reason is that the don´t need feedback for a suitable operating point.
For detailed information and explanation please have a look on the attached tutorial.

My general recommendation: Before using a specific linear IC (like OTA) it wouldn´t be bad to gather some information on the principal function of the unit.