Hi all ,

I need to design a non-inverting amplifier to drive the input of a 50 ohm spectrum analyzer. This is because I want the spectrum analyzer to have very high input resistance.

I thought of using an opamp non-inverting amplifier. However I need to know what is theoutput impedance of the circuit, so that I can match it to 50 ohm ( for maximum power transfer). What is the equation for the output resistance of such a circuit?

Also, if anyone knows of a better solution to my problem, suggestions are more than welcome .

Thanks in advance.

 

The amplifier output impedance is Rout = Ro * ACL/Ao, where Ro is the Op Amp output resistance, ACL is the non-inverting amplifier gain and Ao is the Op Amp open loop gain. Therefore, Rout is very small. So, connect a 49.9 ohms in series with your Op Amp output and drive the input of your spectrum analyzer. You will have a good impedance match.

 

Thanks a lot Adrian. Allow me one more question please. If ACL is unity, and I place a 49.9 ohm resistor in series with the output, my signal voltage at the analyzer input will be half the voltage at the opamp input. Is that correct?

Will I get the voltage at the analyzer input eqaual to the voltage at the opamp input if I make ACL = 2?

Thanks.

 

Thanks a lot Adrian. Allow me one more question please. If ACL is unity, and I place a 49.9 ohm resistor in series with the output, my signal voltage at the analyzer input will be half the voltage at the opamp input. Is that correct?

Yes, correct.

Will I get the voltage at the analyzer input eqaual to the voltage at the opamp input if I make ACL = 2?

Correct again. Good luck with your project.

 

Most opamps have a full output frequency limit of 100kHz. A 741 opamp has a full output frequency limit of only 9kHz.

Most opamps can drive a load resistance that is no lower than 2000 ohms but a few can drive no lower than 600 ohms.

An opamp cannot drive a 100 ohm load unless the output voltage is low so that the output current is a peak of only 20mA. If the peak current into 100 ohms is 20mA then the 100 ohms has an RMS voltage of 1.41V and the 50 ohms input of the spectrum analyser gets a max of only 0.71V RMS.

 

Most opamps have a full output frequency limit of 100kHz. A 741 opamp has a full output frequency limit of only 9kHz.

Most opamps can drive a load resistance that is no lower than 2000 ohms but a few can drive no lower than 600 ohms.

An opamp cannot drive a 100 ohm load unless the output voltage is low so that the output current is a peak of only 20mA. If the peak current into 100 ohms is 20mA then the 100 ohms has an RMS voltage of 1.41V and the 50 ohms input of the spectrum analyser gets a max of only 0.71V RMS.

The great trade off. There are power amplifiers from Texas Instruments that can deliver 10A continuous DC current, but they will be limited in rail performance, optimally require dual supply, and the GBW is not tremendously high.
APEX should also have power amplifiers as well.

I am not sure about spectrum analyzers, but one can add an offset for vector network analyzers like the Rhode and Schwarz when the impedance is not 50Ω. I have never tried, but the Rhode and Schwarz VNA should be able to be configured for any type of impedance. There should be BNC type of termination resistances on the back. You may want to check the spectrum analyzer for such a configuration.

 

Audioguru, you are right, but I assumed that SimonAB has a suitable Op Amp for his application. There are Op Amps out there that can drive such loads. For example, Analog Devices' AD817 can drive a 100 ohm load and has a bandwidth of 50 MHz. I used it many times in my designs. Its output current is minimum 50mA with a short circuit of 90mA, so you can have a guaranteed 5V amplitude on 100 ohms, 2.5V amplitude at the spectrum analyzer input.

 

I am going to use the AD8057 opamp. It has a bandwidth of up to 325MHz. Also in the data sheet there is that at a load of 150 ohms the ouput voltage swing is between -4 to +4 volts. This should enough for my application.
Really appreciate your help. Thanks a lot .