Complementary voltage generator

[Sezi]

Hello,
I want to generate a complementary analog voltage out of another voltage. I tried the following circuit you'll find in the first link and the in-out characteristic is in the second link. But i want both a linear and a peak-to-peak(almost) variation at the output voltage. What do you think about this schematic, what can i use instead?

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Sezi

 

[Davood Amerion]

I think your circuit is not good.
it is better to use one op-amp in inverting configuration.
connect pin '+' of op-amp to middle of max. output voltage swing, through voltage divider( 2 resistor).
pin '-' go to output through 22K resistor.
and same pin ('-') connect to input through 22K.

Regards
Davood Amerion

 

[Sezi]

hi, I couldn't understand the configuration

 

[sunking]

yes, use an inveter opa

 

[Sezi]

Hello Davood and sunking,
I have designed an opamp as you have suggested and here is the circuit schematic and waveform:

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They don't look much different, only it's more linear.. i want it to be rail-to-rail :|

Added after 39 minutes:

And also, i want to connect this generator to the output of the chargepump and a passive lpf, and if i do that, the current goes through the resistive divider over the opamp instead of charging/discharging the lpf.. i could well use a buffer in between, but the first circuit(two transistors) is slightly simpler than all this, you see..

 

[Davood Amerion]

Hi Sezi,
if you want to generate accurate negative ( analog )signal from analog input, you can use op-amp.
but if linearity is not important, you can use your circuit.

For rail-to-rail op-amp you can use (eg.) OP162, OP262, OP462,...
R1 must equal to R2.
and choose them according to input impedance.

Regards
Davood

 

[Sezi]

Ok i decided to use the first circuit, i only added another pmos transistor parallel to the pmos, with its gate connected to its drain (saturated pmos). it slightly improved the characteristic. still not so linear, but the vco works fine with those inputs anyway..
thank you guys for your help :!: