TLV074 ic amplifier section gets oscillating - No output???

savin saju23

Other Parts Discussed in Thread: TL074

dear all ,

amplifier section gets no output when gain of second and third stage of amplifier gain is increased above value 2::( GBW of tlv074 = 100)....THESE STAGES ARE ROUNDED IN the below schematic....input to first stage of opa-amp is 1 mV p-p sine wave at 10KHz...

Tested conditions:

gain1 = 50

gain 2 = 5

gain = 2

output = oscillations only - no signal

over 9 years ago

0 Diego Melendez over 9 years ago

TI__Guru 57800 points

Hi Savin,

Welcome to E2E!. TLV074 looks like is not an available part number, Could you please provide the full name of the device in question?

Thanks!

Regards,

-Diego Meléndez López
Audio Applications Engineer

0 savin saju23 over 9 years ago in reply to Diego Melendez

Prodigy 60 points

part number = TL074

0 Ron Michallick over 9 years ago

TI__Guru**** 151466 points

Savin,

I did not see the schematic attachment. Can you try again?

Are all three stages oscillating? If so try separating the stages to see if the oscillation stops. You may have a unintentional feedback from output to input.
Oscillation usually occurs when there is too much capacitive load. Also check the common mode voltage; VCC- + 4V < INPUT < VCC+

0 savin saju23 over 9 years ago in reply to Ron Michallick

Prodigy 60 points

attched the schematic file

0 Ron Michallick over 9 years ago in reply to savin saju23

TI__Guru**** 151466 points

Savin,

The supply voltage(6.6V) is quite low for TL074. 10V minimum is recommended.
The common mode voltage is 3.3V (greater than pin VCC-). Greater than 4V is recommended
It also appears the the supply voltage are the wrong polarity or wrong pins.

The total gain is very high for a DC coupled amplifier. I suggest AC coupling the stages. The final stage is probably stuck high or low.
The TL074 won't provide a rail to rail output, there may be output clipping.

0 savin saju23 over 9 years ago in reply to Ron Michallick

Prodigy 60 points

i have corrected mistakes in schematics ....given supply is +-5V...and capacitor is placed between two amplifier stages...Now the problem is: still oscillation exists at high gain stages..

Eg: gain1 = 100

gain 2 = 50

gain 3 = 2

at this condition output of op-amp is still oscilIATING??

GBW=3MhZ is enough for this requirement???

0 Ron Michallick over 9 years ago in reply to savin saju23

TI__Guru**** 151466 points

3MHz is good enough. 100 gain * 10kHz = 1MHz.

0 savin saju23 over 9 years ago in reply to Ron Michallick

Prodigy 60 points

Hi,

As shown in fig. when we are cascading the op-amps with one another in three stages, the effective GBW of op-amp is degraded..i heard that there is some effect on cascading amplifier stages....Is this condition decrease GBW of whole system shown in schematic???

If it is right, what is recomemded gain levels in three stages differently??

0 Ron Michallick over 9 years ago in reply to savin saju23

TI__Guru**** 151466 points

Savin,

Each amplifier has it own bandwidth. Each stage is its own amplifier with an input and an output.
Oscillation means feedback becomes in phase before loop gain drops to unity (0dB).
Multi stage gain must have careful layout to ensure parasitic feedback does not cause an unwanted feedback.

At what frequency does the circuit oscillate? If the oscillation similar to a sine wave or does it clip?

0 savin saju23 over 9 years ago in reply to Ron Michallick

Prodigy 60 points

output of amplifer producs a sine wave of 240KHz frequency...

Where the actual frequency of operation is 5 - 30KHz sine wave...

how ti eliminate this one???

i learned that when cascading op-amps, there is a degradation in op-amp's GBW .....does it limits max gain that can be given to op-amps??? is this cause for osccilatting??

sharing a link for ur refernce

0 Ron Michallick over 9 years ago in reply to savin saju23

TI__Guru**** 151466 points

Savin,

Try adding a capacitor on each stage from output pin to inverting input pin. This will reduce high frequency gain and stop oscillation.
Choose capacitors using this formula. C = 1/(6.28 * R * F) where R is the feedback resistor and F is 60kHz.

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TLV074 ic amplifier section gets oscillating - No output???