Buffer between high pass and low pass filter when making a bandpass filter?

Question

For a passive filter stage, I want to combine a high pass filter with a low pass filter to create a bandbass filter. Can I just make a simple RC circuit for each and connect them together? Or would we need a buffer between the two. The low and high cutoffs are 625 kHz and 1.535 MHz. This is for the front end of a simple AM receiver tuned to one frequency of 1.08 MHz.

Answer 1

RC circuits are a very bad idea at the front end - unless you have a special application where you really don't care about noise and sensitivity, perhaps the receiver is within a few miles of a high power transmitter.

One precedent for a deliberately insensitive receiver is the "Volksempfanger", designed to be incapable of picking up distant stations such as the BBC.

If you genuinely want to tune one frequency only, use an L-C tuned circuit - traditionally called a "tank" because it stores RF energy in a resonant circuit with no resistor to act as a loss mechanism or noise source.

It will also have the benefit over the R-C approach of reducing other strong signals within the AM band which could cross-modulate and interfere with your wanted signal. Unless you can guarantee the wanted signal is stronger than all other transmissions in the AM band (because you're near the transmitter) this is probably the biggest problem you will face in making a receiver that works well.