Using op amp TL084C as a voltage follower, unity gain (pin 1,2 and 3 and +- 12V supply) and found that the output is attenuating after around 200kHz as against the number in data sheet, what could be the possible reason

[PChaw.1]

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[Peter BENSCH]

Welcome, @PChaw.1​, to the community!

Each opamp has important parameters to consider, including the gain bandwidth product or the maximum peak-to-peak voltage vs load and frequency. However, you must not confuse this bandwidth with the maximum working frequency. You can also see the voltage drop in the data sheet in fig 2 and 3, which is also dependent of the load resistance.

Which frequency range do you want to process?

What is your application?

If necessary, please read the chapter on operational amplifiers in books such as The Art of Electronics by Paul Horowitz and Winfield Hill.

Regards

/Peter

[PChaw.1]

Hi Peter BENSCH

Thanks for your response.

As indicated earlier, the board using TL084C was designed for eight channel, with the attached schematic. Application is to use it for sinusoidal input signal of around 200kHz frequency with the gain up to 7 and input of 1Volts rms.  

The board had two issues; 

1. For the output, beyond 3kHz, it was showing zero cross over distortion. Increasing the value of load resistance to 1.5MOhms (earlier it was 10kHz) had minimized this distortion and therefore not indicated in earlier mail.

2. Before and After the above changes, the output was also attenuating after 25kHz. 

To check the second point,(during the tests mentioned below the input was 1V rms) the circuit of schematic attached was tested separately on breadboard and in-circuit, as two separate op amps. Please refer the schematic attached; 

In first case only one op amp was used as voltage follower with unity gain, the output voltage attenuating after the frequency of 200kHz.  

In the second case the complete circuit using two op amps were used and the output was attenuating after 25kHz. With the components values shown in the schematic attached, the gain of the circuit is 7 and the input voltage is 1V rms.

Can this op amp be used for the above application and or up to what frequency and gain?

Looking forward for your response

Regards

Pradeep

[Peter BENSCH]

The TL084C is already a rather old opamp whose bipolar output stage is not yet capable of rail-to-rail swing. In addition, the output also contains series resistors, which explains the load dependence of the output voltage.

DC: the first mentioned point is important for the output range, because the datasheet gives an output voltage swing of typ 12V for a 2kohms load and a VCC=±15V, i.e. 3V voltage drop in each output stage. If you now work with VCC=±12V and feed in 1Vrms at k=7, you get an output voltage of 1V*sqrt(2)*7 = 9.9V, which offers only 2.1V reserve. At higher loads you will observe clipping.

AC: What attenuation did you measure with the voltage follower and the amplifier at the different frequencies (1kHz, 10kHz, 25kHz, 200kHz) compared to DC or a low frequency?

Regards

/Peter

[PChaw.1]

Hi Peter BENSCH

For a time being, let us consider only a voltage follower stage, where, the input is kept only at 1V rms (and also tried at 1V peak to peak) with the different frequencies, from 50Hz to 500kHz or so. The output, as expected is same as input, up to 200kHz and beyond 200kHz the output drops/attenuates with the slope of 20db/decade.

The output wave is observed as sine wave, no clipping of waveform and not distorted one, even at higher (>200kHz) frequencies, but with the attenuation, even as voltage follower.

If, for the unity gain, the maximum frequency of operation is known, the other things can be interpreted or worked out, and as per data sheet, this number should be around 3MHz and I am getting this as 200kHz.

Regards

Pradeep

[Peter BENSCH]

OK, in fact, with a follower with the TL084C a frequency of 1MHz should be reached without any issues, a significant drop should only be seen from about 2...2.5MHz, as this can also be checked with a simulation. If you actually observe a drop starting already at 200kHz, something is wrong and everything should be double-checked again: the signal quality of the source, the circuit (power supply etc), the design of the system, the component itself (original or counterfeit), occurrence of the behavior with several test setups?

Regards

/Peter

[PChaw.1]

Hi Peter BENSCH,

Thanks,

Checked several times and it seems to be the issue of "lot of supply of these chips" (unbelievable, but its fact), as for the similar tests on the ICs (TL084CN) from the earlier batch of supply, I am getting better response up to 2MHz, meeting our requirements.

Regards

Pradeep

[Peter BENSCH]

Great.

If the problem is resolved, please mark this topic as answered by selecting Select as best under your preferred answer. This will help other users find that answer faster.

/Peter