Consider the following system-level diagram of an AM receiver:

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I'm currently focusing on the baseband amplifier stage, which is used to amplify the audio signal. Its corresponding schematic is:

enter image description here

The amplifier shown in the schematic is a TL084. I have few questions about the design of the baseband amplifier:

1) What is the purpose of C1 (1 micro Farad)? I believe it is included to act as a high pass filter, that is, it keeps any DC signals from affecting the circuit.

2) What is the purpose of R1 (150,000 Ohms)? Is R1 the input resistance of the circuit? It makes sense to have a large input resistance since we will be connecting another stage before the baseband amplifier stage.

3) What is the gain equation of the baseband amplifier? Is it okay to assume ideal operational amplifier conditions for this analysis (virtual short between the two inputs of the circuit and no current going into the amplifier)? If it is, then it is easy for me to find the gain equation.

4) Choose values for R2 and Rpot (0 to 10,000 Ohms) that will achieve an in-band gain of 150 V/V to the nearest 5% accuracy.


1 Answer 1

  1. C1 is a DC blocking (coupling) cap exactly as you suspected.
  2. R1 is to provide a DC path for the bias current for the input stage of the op-amp.
  3. The gain will (1+R2/Rpot) with a negligible error due to finite open loop gain of the amplifier.
  4. 1+10000/Rpot = 150, so Rpot = 68.1 ohms. A=146.8 which is within 5% of 150.

Note that typical gain-bandwidth for this amplifier is 3MHz, meaning your typical closed-loop bandwidth for this amp will be 20KHz. Likely fine for an AM receiver.

  • \$\begingroup\$ Would you mind explaining what you mean by "R1 is to provide a DC path for the bias current for the input stage of the op-amp"? Thank you! \$\endgroup\$ Commented Sep 14, 2015 at 19:53
  • \$\begingroup\$ If R1 is not there, what will be the DC voltage at the + input of the opamp ? Answer: it is undetermined as C1 blocks DC so that + input would be floating. Never leave inputs floating ! The opamp needs a proper DC level at every input. R1 sets the DC level to 0 V and the opamp will be happy. \$\endgroup\$ Commented Sep 14, 2015 at 20:49

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