Antenna input becomes zero when connected to a op amp amplifier

MikeML

Joined Oct 2, 2009
5,444
There is no information on the signal. It is a plain 1kHz signal that is being captured. EMF. I will probably be using an RC Low pass filter of 0.5-1Hz to make it a smooth line on its average.
So the "antenna" is a wire between a signal generator and the input of the opamp?

Connecting a 14" piece of wire to the input of a high-input impedance amplifier is only going to pick up electrical noise and a bunch of 50/60 Hz powerline stuff. What is the application? What do you expect to see if you filter the powerline and its harmonics?
 

Thread Starter

Jayvin

Joined Mar 8, 2015
47
So the "antenna" is a wire between a signal generator and the input of the opamp?

Connecting a 14" piece of wire to the input of a high-input impedance amplifier is only going to pick up electrical noise and a bunch of 50/60 Hz powerline stuff. What is the application? What do you expect to see if you filter the powerline and its harmonics?
Indeed. I am interested in the noise that is captured by the antenna. I need the system to tell me if i am going towards or away from the transmitter.
 

Papabravo

Joined Feb 24, 2006
21,159
There is no information on the signal. It is a plain 1kHz signal that is being captured. EMF. I will probably be using an RC Low pass filter of 0.5-1Hz to make it a smooth line on its average.
It sounds like a prescription for zero output to me.
Q: What is the long term average of a 1 kHz. sinewave
A: Zero

Q: What sort of waveform is across the load resistor of the transmitter?
A: ??
 

Thread Starter

Jayvin

Joined Mar 8, 2015
47
If you see the signal that i posted, you would find that the signal is not really symmetric. Also passing the signal through the rail to rail op amp with only a positive voltage and ground would act like a diode and cut out the negative portion of the signal. well from what i have observed using experiments.

And for the transmitter, i have not really designed it.. I tried doing that, and i found that i received the signal at the antenna. I did put a 470Ohm resistor there. But i have to properly redesign it so that i do draw that much current and burn the microcontroller.

As i said, my input was a square wave, i should have used a sine wave, i am still looking for a wave to do that. At this point, i am concentrating on getting the input through the op amp only.

The transformer is of ratio 1:40 50Hz transformer. An there is a problem of using a square wave on the transformer, it get converted to an impluse output, same as that of a differentiator.
 
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crutschow

Joined Mar 14, 2008
34,285
I asked "Is the signal shown in your original post measured directly from the antenna?
If so what it look like when you connect the op amp?
At 1kHz your circuit should work. Does it amplify a test signal?"
Yeah. directly from the antenna. I did not take the picture as i didnt even receive any signal. it was about zero.
If i use a 1kHz signal from a signal generator, the amplifier works.
I don't understand your answer. :confused:
If you didn't take the picture, then who did take it and where was it from?
 

MikeML

Joined Oct 2, 2009
5,444
Back to the non-inverting opamp schematic you showed. If you just hook the "antenna" to the non-inverting input, there is no bias path to the input, and the opamp will just go the rail.

Solution is to have resistor shunting the input to a voltage which is half-way between the opamp's Vdd and Vss

Furthermore, you are going to need a large gain, likely >1000; not 10 as stated.

I would be looking for a low-noise, low-input bias current, j-Fet input opamp.
 
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Thread Starter

Jayvin

Joined Mar 8, 2015
47
Back to the non-inverting opamp schematic you showed. If you just hook the "antenna" to the non-inverting input, there is no bias path to the input, and the opamp will just go the rail.

Solution is to have resistor shunting the input to a voltage which is half-way between the opamp's Vdd and Vss

Furthermore, you are going to need a large gain, likely >1000; not 10 as stated.

I would be looking for a low-noise, low-input bias current, j-Fet input opamp.
Virtual ground for the op amp? Would a BF245A be good enough?


Yeah. probably a gain more than 100 i guess, but i wanted the preamp to be of gain about 10 or around this value. because i willl be amplifying futher again using the op amp.
 
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