2
\$\begingroup\$

I have an input sine wave at 300mV peak to peak. I want to filter this sine wave to eliminate noise and distortion.

I am planning to use the LMx58-N op-amp family, powered using a single supply: 3V to 32V.

Circuitry proposed will be based on the Multiple-Feedback Topology, as shown:

enter image description here

Considering the input signal to be at 100kHz +-10%, I want the filter to work adequately and without any damage to the op-amp. I also want to adhere to good electronic design principles.

Can I pass the signal directly into the above circuit, or should I bias the input signal ?

Any tips and/or suggestions would be greatly appreciated.

\$\endgroup\$
  • 1
    \$\begingroup\$ Read the datasheet! Section 6.1 Absolute Maximum Ratings gives the absolute minimum input voltage. But that's to avoid damage, not necessarily for practical operation. \$\endgroup\$ – Finbarr Oct 28 '18 at 16:20
  • 1
    \$\begingroup\$ Some can. This one can't. \$\endgroup\$ – Hearth Oct 28 '18 at 16:21
  • 3
    \$\begingroup\$ This circuit is DC-blocked by Cin. Thus DC components do not matter. \$\endgroup\$ – analogsystemsrf Oct 28 '18 at 16:46
3
\$\begingroup\$

The bias is signposted already as Vcc/2 (mid-rail supply). In the quiescent state VOUT will be at Vcc too so you may need a DC-blocking capacitor on the output too.

R1 and R2 are a potential divider and form an input attenuator.

Read the datasheet carefully to understand what the maximum output voltage swing can be when operating at low voltage at the load resistance value you are proposing.


enter image description here

Figure 1. Internals of the ancient 741 opamp. Source: Wikipedia.

From the internal schematic of the 741 op-amp it should be clear that the output can source current from the \$ V_{S+} \$ rail via Q14 or sink current to the \$ V_{S-} \$ rail via Q20. The arrangement of the output transistors determines how close to the supply rails it can get. The 741 is an ancient op-amp and is of very low quality by today's standards. It couldn't get within 2 V or more of the supply rails and would be totally unsuited to low voltage operation.

Are you referring to the voltage gain that can be achieved when operating at the proposed voltage and load resistance?

So, I'm not referring to the voltage gain, but rather the voltage swing at the output. The maximum input signal it could tolerate would be \$ V_{IN} = \frac {V_{OUT}}{GAIN} \$. For example, let's say your op-amp is able to swing 3 V from output minimum to output maximum and the gain is 10 then the maximum input signal before clipping would be 0.3 V.

\$\endgroup\$
  • \$\begingroup\$ Many thanks for your suggestions and pointers. I am a beginner when it comes to electronics and do not fully understand your last sentence. Are you referring to the voltage gain that can be achieved when operating at the proposed voltage and load resistance? \$\endgroup\$ – Rrz0 Oct 28 '18 at 16:37
  • 1
    \$\begingroup\$ Yes. Operational amplifier circuits often have outputs that can not come close to the power supply rails. An output that can't drive closer than two diode drops is common (because of a double emitter follower in the output stage). This often comes as a surprise to the newbie (and sometimes even the experienced). There are op-amps that can swing the output from rail to rail -- but you have to find them. \$\endgroup\$ – TimWescott Oct 28 '18 at 18:03
  • \$\begingroup\$ @Rrz0: See the update. \$\endgroup\$ – Transistor Oct 28 '18 at 21:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.