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Small signal amplifier

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saleheen

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lm6628

Hi all, I need to amplify a small signal which is about 500 mVrms (an AM signal) to about 2 to 4 Vrms.. What kind of amplifier ICs I could use? I tried using LM741 but it is not able to do the job. I need to build up this circuit urgently. Please help.. Thanks
 

741 signal amplifier

saleheen said:
I tried using LM741 but it is not able to do the job.

What does this mean - "not able to do this job" ? How did you figure this out ?
What is the frequency of your signal ?
 

lm6628 datasheet

The signal that is supposed to be fed to the amplfier is a signal output from Amplitude Modulation IC. It has very low amplitude (about 500 mVrms). I need to feed this signal to a time delay circuit.
When I connect both circuits together, I am not able to get any signal out from the delay circuit. Due to the small amplitude, the signal is lost due to noise. Therefore, I need to amplify it before feeding it the delay circuit. I tried a couple of method to amplify it but I was able to succeed.
 

output coupling for lm741

For my opinion, 500mvolts are not a "very low signal amplitude". Normally, it should not be a problem to amplify it with an opamp - even with the old 741 type.
But, you did not answer both of my questions:
1.) What is the frequency of your signal ?
2.) How did you come to the conclusion "..not able to do the job" ?
What did the amplifier ? Nothing ? Saturation ? How does the circuit diagram of the amplifier look like ?
 

+conclusion about small signal amplifier

1) The frequency is between 36 kHz to 44 kHz. Fc=40kHz, fm=4kHz. Therefore, the signal outputs are fc+fm, fc-fm and fc.
2) The LM741 is only able to generate the same signal. unable to amplifly it.
Initially, I was using LM7121 (as in the attached diagram) but I was not able to get any signal at the output.

Hope you have a better picture of the difficulties I am facing at the moment.
 

saleheen said:
1) The frequency is between 36 kHz to 44 kHz. Fc=40kHz, fm=4kHz. Therefore, the signal outputs are fc+fm, fc-fm and fc.
2) The LM741 is only able to generate the same signal. unable to amplifly it.
Initially, I was using LM7121 (as in the attached diagram) but I was not able to get any signal at the output.

1.) Frequencies around 40 kHz are too high for a 741 type opamp. Try any other type with a transit frequency of at least 5 MHz - and watch the slew rate SR ! For example, the 741 can swing to its maximum output only up to app. 10 kHz
(SR=0.5 V/usec).
That means you should select an opamp with a slew rate better than several V/usec.

2.) Why only 500 ohms for the resistors in the feedback path ? It´s better to use some kohms.
 

    saleheen

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That value was recommanded in the datasheet of LM7121
 

hi
what you want is '500 mVrms (an AM signal) to about 2 to 4 Vrms.. '.
but the value from lm7121 is just 2*500 mVrms=1v.
Vo=((Rf+Rg)/Rg)*Vi.
YOU can use 2.2Kohms for Rf.
good luck!
 

    saleheen

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Hi flyingSky, Actually I am not able to get any signal from the buffer.. The signal from the AM is lost probably due to noise. My project supervisor told me to increase the amplitude to about 2 to 4 Vrms before feed the AM signal to the buffer(LM7121). Which mean I may need another buffer to do this. LM7121 is actually a buffer which give high input impedance and it's recommanded by LM6628 (the time delay IC).
 

saleheen said:
Hi flyingSky, Actually I am not able to get any signal from the buffer.. The signal from the AM is lost probably due to noise.

It is really not possible that a 500mV signal disappears due to noise.
There must be another cause. Do you use split (dual) power supplies ?
Did you try a simulation of your circuitry ?
 

Yes I use dual supplies. -+12V for the AM and -+5V for the buffer and delay circuit.
 

saleheen said:
........Actually I am not able to get any signal from the buffer..

Please, be more specific: What means "not able to get any signal..." ?
What do you measure at the output ? What dc level ? Perhaps the IC is damaged ?
 

When I test the circuit using a oscilloscope, I don't get any signal. As in, good as nothing connected to the oscilloscope.
 

saleheen said:
When I test the circuit using a oscilloscope, I don't get any signal. As in, good as nothing connected to the oscilloscope.

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.
I see no other possibility.
 

When I replace the AM signal to a functional generator signal, I am able to get the output signal from the buffer. Is it due to some interface between the AM and the buffer ?

Added after 3 minutes:

Can I use a small signal amplifier to amplify the signal before feeding it to the LM7121 buffer?
 

saleheen said:
When I replace the AM signal to a functional generator signal, I am able to get the output signal from the buffer. Is it due to some interface between the AM and the buffer ?

In this case, I am afraid you did not use a coupling capacitor between AM output and buffer input. Am I right ?
 

    saleheen

    Points: 2
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What should be the value of the capacitor?
 

The coupling cap should have a resistance at the lowest frequency which is negligible.
I would guess that app 1uF is OK.
 

If a coupling capacitor is used then the input of the opamp needs a resistor to ground to set the DC voltage at 0V.
 

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