How Can a Buffered Band Pass Filter Be Designed with an Infinite Resistive Load?

  • Thread starter Thread starter Idyllic
  • Start date Start date
  • Tags Tags
    Infinity Load
Click For Summary

Discussion Overview

The discussion centers around the design of a buffered band pass filter with an infinite resistive load. Participants explore the implications of using an infinite resistance load in filter design, particularly focusing on the role of buffering and the effects of load resistance on circuit behavior.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how to design a buffered band pass filter with an infinite resistive load, suggesting that infinite resistance implies an open circuit.
  • Another participant explains that assuming an infinite resistance load simplifies circuit analysis, as it keeps the low pass time constant unaffected by load resistance.
  • A participant notes that buffering isolates the filter's resistances and capacitances from other circuit elements, providing an effective infinite load by drawing no current.
  • Concerns are raised about the circuit's functionality without a load, with a participant stating that a capacitor would charge to a voltage and remain there indefinitely.
  • There is a clarification regarding the circuit's components, with a participant suggesting that the buffer amplifier is crucial for proper operation.
  • Another participant describes the circuit's structure, indicating that the buffer ensures only two time constants are involved, preventing the resistances from affecting the charge rates of the capacitors.
  • One participant asserts that the circuit functions as designed but will require adjustments to the cutoff frequency if a load is attached.
  • Suggestions are made to use additional buffers to maintain the desired filter characteristics when a load is present.
  • A later reply emphasizes that the absence of a load resistance is what effectively creates an infinite load, and that introducing a load would alter the filter's gain and frequency response.
  • Two strategies are proposed for dealing with non-infinite load resistance: designing the filter around a known load or using a second buffer to isolate the load from the filter.

Areas of Agreement / Disagreement

Participants express differing views on the implications of using an infinite resistive load and the necessity of a load in the circuit. Some agree on the role of buffering, while others challenge the idea that an infinite load can be effectively implemented without consequences.

Contextual Notes

Participants highlight the importance of load resistance in filter design, noting that it can significantly affect the circuit's performance and characteristics. The discussion reveals uncertainty regarding the practical implementation of infinite resistance in real-world applications.

Idyllic
Messages
13
Reaction score
0
Hello,

I have a question. How do you design a buffered band pass filter with a resistive load of infinity?

I have a feeling that a resistor with a resistance of infinity is an open circuit. Although how can this be implemented in a type of filter seen in the attachment?


Thank you.
 

Attachments

  • bufferedbandpassfilter.jpg
    bufferedbandpassfilter.jpg
    7.1 KB · Views: 608
Engineering news on Phys.org
The assumption of an infinite resistance load (also know as "no load") is purely to make the analysis of the circuit easier, as it means that the low pass time constant is unaffected by the load resistance. You can achieve this by simply placing a second buffer (voltage follower) between the output and the load.
 
One point of 'buffering' a filter is to isolate the resistances and capactiances that constitute the filter from other elements in the circuit which may not be defined and which might affect the frequency response. A load resistance of 'infinity' is what a good buffer will provide (no current will be taken). There is nothing more to it than that, I think.
 
Your circuit will not work without a load.
C2 will charge up to a voltage and stay at this voltage forever.
 
Carl Pugh said:
Your circuit will not work without a load.
C2 will charge up to a voltage and stay at this voltage forever.

His drawing is not very clear, but I think that's a buffer amp in the middle, and not a diode. If it's an amp, it will work okay. If it were a diode, it would have the problem that you mention.
 
The left hand bit is the high pass (working into a high impedance) and the right hand bit is the low pass (fed from a low impedance) the buffer (in between them) ensures that there are only two time constants involved. Without it, R1 and R2 would both have an effect on the charge rates of both C2 and C1.
 
Yeh sorry about that it is a buffer. So that buffer makes have a resistive load of infinity? there's nothing else to add?
 
The circuit is fine. It works as is.

All it means is that the designed cutoff will change if you attach a load. Simply adjust R2 once you have a finite load impedance and all is well.
 
Or follow with another buffer. Many packages have more than one amp in them, in any case.
 
  • #10
Idyllic said:
Yeh sorry about that it is a buffer. So that buffer makes have a resistive load of infinity?

No that is not correct. That is not what people have been telling you.

The thing (in your given circuit) that makes the load resistance infinity is the absence of a load resistance (connected in parallel with c2).

If you were to place a load resistor in that location then it would alter both the gain and the frequency response of your filter. The load resistance effectively becomes an integral part of the filter circuit and modifies it's behavior.

In such cases where there is a load resistance present (that is, load resistance is not infinity) then we have two choices.

1. If the load resistance is well know and consistent enough then we can design the filter around the load resistance, so that it achieves it's a specific gain and frequency response with the load resistance included as part of the filter specifications (effectively as an integral part of the filter itself).

2. We can use a second buffer connected at the output of the filter (after c2) to isolate the load from the filter.
 

Similar threads

Confusion on: High Pass Filter at Op Amp Input
  • · Replies 6 ·
Replies
6
Views
3K
Question about Designing EMG data acquisition
  • · Replies 51 ·
2
Replies
51
Views
9K
Home Brew RF dummy load showing reactance at higher frequencies
  • · Replies 24 ·
Replies
24
Views
10K
Finding the resonant frequency of this "discriminator" demod circuit.
  • · Replies 2 ·
Replies
2
Views
3K
Wavelet transform (CWT and DWT)
  • · Replies 4 ·
Replies
4
Views
3K
Op-amp Oscillator Circuit Design (10-50 MHz)
  • · Replies 6 ·
Replies
6
Views
3K
Passive and Active Low Pass Filters
  • · Replies 5 ·
Replies
5
Views
2K
Op-amp compensation of cascade fudges audio signal oddly
  • · Replies 3 ·
Replies
3
Views
2K
Can You Easily Design a Variable Band Pass Filter in the MHz Range?
  • · Replies 4 ·
Replies
4
Views
2K
Line filter, proper LTspice simulation
  • · Replies 15 ·
Replies
15
Views
7K