Differential Amplifier for modulator input

Thread Starter

rivaldicahya

Joined Jun 13, 2019
2
hey, i need your help for making a differential amplifier using op-amp LM741.

I'm currently working on my finals creating an OQPSK modulator using IC AD8345. in the datasheet it is said that the input should be differential amplified and dc biased by 0.7 volt for 1.2Vpp (the maximum input for the modulator).

my bit stream for the input is currently 5-6vpp with 600KHz frequencies , and i need to convert it as i mentioned above. (in the datasheet, it is mentioned that diff. amplifier such as AD8132 can be used but it's hard to find in my country. nb: i bought the modulator from china and forget to buy the diff. amplifier).

pls help me, your answer is really appreciated.
 

wayneh

Joined Sep 9, 2010
16,102
...my bit stream for the input is currently 5-6vpp with 600KHz frequencies ...
You had me until I got to that line. The ancient 741 can't come anywhere near that high a frequency. I'd worry about it at 6kHz let alone 600kHz.

Even a "generic" modern op-amp will struggle. You need a specialized "fast" op-amp.

Sorry if I've misunderstood your needs.
 

danadak

Joined Mar 10, 2018
3,577
The AD8132 it is asking for is a 350 Mhz GBW OpAmp, so its clear the 741
would never work in that application. And its diff in and out to boot.

I would search on digikey web site for possibilities -

https://www.digikey.com/products/en/integrated-circuits-ics/linear-amplifiers-video-amps-and-modules/737?k=&pkeyword=&sv=0&pv41=283&pv2153=u350MHz&sf=0&FV=ffe002e1&quantity=&ColumnSort=0&page=1&pageSize=25

And when you do this design board layout very state of the art to handle these
bandwidths.

Regards, Dana.
 

Eric Matias

Joined Jun 14, 2019
4
1. PWM can be used to convert a digital value to an analog voltage by using the digital value to set the Duty Cycle. When the PWM output is sent through a Low Pass Filter, the output is a DC voltage that is proportional to the digital value. Drawbacks are: a tradeoff between ripple voltage and conversion delay (or reduced rate of change) -- this can be reduced by lowering the PWM period -- i.e. the higher the frequency, the easier it is to filter out the "carrier" frequency, and the quicker the filter response can be.

2. Because multiplication can be done by converting the values to exponential form, then adding the exponents, and then taking the log of the value. This is an old Analog Computer trick, and was done because it's a lot easier to ADD voltages with Op-Amps than it is to multiply voltages. And, it's fairly easy to do the linear to exponent and back to linear conversions. It just takes a lot of components
Also, probably good to use higher voltages and dual supplies, and as such is probably WAY over kill for your application
But, it's a nice bit of esoteric information, no?
 

Thread Starter

rivaldicahya

Joined Jun 13, 2019
2
You had me until I got to that line. The ancient 741 can't come anywhere near that high a frequency. I'd worry about it at 6kHz let alone 600kHz.

Even a "generic" modern op-amp will struggle. You need a specialized "fast" op-amp.

Sorry if I've misunderstood your needs.
well thank you for replied my thread.
say i want to change the freq to the little one like about 4.8KHz, can i still use the 741? also can you help me how to create it?
 
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