low frequency flat amplifier

[manasiri]

I need a circuit for an amplifier with the following specifications:

1- frequency band : 25 kHz-125 kHz
2- Gain: 10 dB (not important) but flat response in the band.
3- mathing: input=50 ohm , output= 50 ohm (for coaxial cable connection to antenna and ...)

i tried with OP07 (operational amplifiers) but it has problem.
thanks.

 

[FvM]

Sounds very strange to use 50 ohm matching for kHz "antenna" cable. (If you are not using km antenna dimensions, the antenna would be usually designated e.g. a coil).

OPs are generally usable, but the signal level and noise specification should be known to select a suitable type. Also a single transistor stage is good for 10 dB gain, of course.

 

[manasiri]

we need to detect a signal in the band that is not a sound signal.

we work with spectrum analyzer then we need 50 ohm connections to instrumentation and measurements.

 

[LvW]

i tried with OP07 (operational amplifiers) but it has problem.

The first step to solve a problem is to locate and to describe it.
Therefore, what is the problem ?

 

[manasiri]

we search a weak (low level square wave f=35kHz to 125 kHz)
signal level : -50 dBm to -90 dBm .

we must find , amplify and extract the signal .

 

[LvW]

If my calculation is correct that means for - let´s say -60dBm- a voltage of app. 200 mikrovolts across 50 ohms. Thus. as FvM mentioned, noise spec has to be considered for the opamp !

 

[manasiri]

I use a good differential amplifier for the signal amplifiyng but output signal damaged. if level lowers, the signal disapeared.

which amplifiers are good for the task?
if i use LNA?
thanks

 

[LvW]

I use a good differential amplifier for the signal amplifiyng but output signal damaged. if level lowers, the signal disapeared.

Did you use a simple diff. amp ? This gives you certainly not enough gain.
Try to select an appropriate opamp. What means "signal damaged" ???

 

[manasiri]

i used op amp based diff amp.
input is a simple 20 khz (0.5 V p-p) sin wave,
but output signal is noisy random shaped signa.

i think there are low noise amplifiers for low lewel signal amplifications. the lna is placed after antenna.(we use them for MHz frequencies. the minicircuits and miteq companies products are good for high frequency applications )
but in low frequencies i cant find good LNA.

could we reach to noise fiqure of less than 5 and gain of 40 dB in the frequency band( 20kHz-100kHz)?

thank you for attention.

 

[LvW]

i used op amp based diff amp.
input is a simple 20 khz (0.5 V p-p) sin wave,
but output signal is noisy random shaped signa.

So, I assume you have used an instrumentation amp, right ?
And which gain ?
There should be no problem in amplifying 500 mVp-p at 20 kHz. I can´t believe that the output is noise corrupted. Did you test the amplifier carefully ?

 

[manasiri]

thank you , researchers,
Gain needed= 40 dB, and i tested the op amps. thy are 4 opamp in an IC with high frequency working specs(GBW>1MHz).

the final question :
Can you introduce me a device (like MAN-1LN minicircuit LNA) but for my favorite frequency band (20-100 kHz)??

thanks again.

 

[LvW]

As a suggestion, look at Texas Instruments homepage (ti.com) .
There are several instrumentation amps listed - together with application notes.

 

[FvM]

I see no reason for OP07 failing in the application, apart from wrong circuit. But because of limied GBW it wouldn't be usable for 40 dB gain and 125 kHz bandwidth.
For the said frequency range, a decompensated OP37 would be better suited, also in terms of 50 ohms noise matching. It should be considered, that OP open loop output resistance is further reducing the GBW when connecting a 100 ohms load with 50 ohms matched output.

The other option is to use a wideband amplifier and build a more universal preamp, that could be used also for LF or even DC. Current feedback OP THS3001 is my standard amp for many above MHz applications. But the low frequency noise performance is better with a low noise OP as OP37. As far as the specification has been revealed, the application isn't noise limited.