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Could be discussed more seriously with a complete schematic. LM7121 as a +2 buffer is surely compatible, with a meaningful supply and an input voltage within the usable range.
Certainly, the chosen opamp will not be the problem.
But you really should try to make some logical investigatians step by step and communicate the results - instead of saying simply "..not able to get any signal".
I repeat part of my reply from August 14th, 13:31 :
Does the scope react correctly when you connect the power supply pin ?
Some dc level (ground, supply, or something between) must exist at the opamp output pin. If not, the line is broken between the output pin and your measuring point.
Therefore again, what means "no signal" ?
Added after some minutes: Try to measure and display the signal at the AM output, after the coupling cap (opamp input) and at the opamp output.
I removed the buffer and connected the AM directly with the delay crcuit. But I added a 100nF cap in between the AM output and the delay input. I am able to get the correct signal after this procedure.
I will get back to this conversation if problem occurs again in the next few days. Guys, thanks for the support especially LvW.. It has been very helpful.. =)
The non-inverting input of the delay unit has a resistor to ground to give it a DC bias voltage of 0V. The Buffer also needs a resistor to ground to give it a DC bias voltage of 0V like this:
I removed the buffer and connected the AM directly with the delay crcuit. But I added a 100nF cap in between the AM output and the delay input. I am able to get the correct signal after this procedure.
I will get back to this conversation if problem occurs again in the next few days. Guys, thanks for the support especially LvW.. It has been very helpful.. =)
The non-inverting input of the delay unit has a resistor to ground to give it a DC bias voltage of 0V. The Buffer also needs a resistor to ground to give it a DC bias voltage of 0V like this:
Without the resistor connected to the ground, what will happen? I see "DC bias" by the resistor is quite straight forward. But why can't the input port of OP-AMP set up a DC link with OP-AMP's rail (power supply or ground)?
Without the resistor, the OP input current will make the input voltage drift, usually beyond the common mode range, the output gets saturated at either positive or negative maximum.
You forgot to say the frequency of the signal you want to amplify. AM radio is at 540kHz to 1.6MHz.
A lousy old 741 opamp has trouble with frequencies that are higher than only 9kHz.
If you make your own RF amplifier then it will probably be overloaded by local AM radio stations.
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