Help with bandpass, highpass, and lowpass filters

Discussion in 'Homework Help' started by patentxnewb, Jan 14, 2015.

  1. patentxnewb

    Thread Starter New Member

    Jan 14, 2015

    I have a homework assignment that asks me:

    1) "Is there any problem with replacing a bandpass filter of a circuit with both a highpass filter and a lowpass filter connected in serial?"

    2) "If there are no problems, why do engineers use a bandpass filter instead of combining a highpass filter with a lowpass filter?"

    3) "Does adjusting the gain of a bandpass filter amplify an input/output signal of that filter?"

    Please help, I'm totally stuck on these questions.
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    Regarding 2.
    Count how many parts you need for purpose made bandpass filter.
    Count how many parts you need for bandpass filter that is made using high pass and low pass filters.
    Compare the two numbers.
    Now assign a price to each part, say one dollar.
    Which bandpass filter is more expansive? The purpose made bandpass filter or the high pass+low pass bandpass filter? There lies the answer to your question.
  3. crutschow


    Mar 14, 2008
    Don't really understand question 3).
    How is the gain being adjusted?
  4. WBahn


    Mar 31, 2012
    What are YOUR best thoughts on these questions?

    What do you think some of the advantages and disadvantages of using a single bandpass filter over a cascade of low and high pass filters might be?

    You need to be a bit more descriptive in question 3. The term "amplify" can either be interpreted as meaning strictly to make larger or it can be interpreted as being more general and just meaning to change the size (possibly even leaving it alone) so that a given "amplifier" could have a gain of 0.5, 1.0, or 1.5, for instance.
  5. patentxnewb

    Thread Starter New Member

    Jan 14, 2015
    thanks for the replies but im really new to circuits. can you guys explain your answers for questions 1 and 2, please.
  6. WBahn


    Mar 31, 2012
    In other words, you want us to do your entire assignment for you without you putting forth any show of effort.

    I'll pass.
  7. Yakima


    Jan 23, 2012
    The key to thinking about these things is considering the effect of frequency on the Xc (reactance) of the capacitor, and the effect this has on either a series capacitance or a parallel capacitance, in a manner similar to resistances in series or parallel. For thinking purposes, see the capacitor as a variable resistance as a function of frequency to an ac signal. Then remember the effect of resistors in series and resistors in parallel. But regardless of series or parallel, remember that as the frequency goes up, the "resistance" of a capacitor goes down. For proof this consider the equation of frequency with respect to reactance (i.e. resistance). If you choose a large capacitor at the emitter bypass then you will dampen the amplitude of the signal at a lower frequency than a small capacitor. A small capacitor is like a large resistance with respect to frequency while a large capacitor is a smaller resistance with respect to the same frequency. Once you get a handle on this and get familiar with it due to usage, the effects of a capacitor will be clear at the outset.

    By choosing the proper bypass capacitor at the emitter, you are specifying your low pass -3dB point. After that you want to create a high pass -3dB point and this is gotten by using a capacitor in series with the input resistance of the transistor. Remember that when a capacitor is in series with a resistance, the overall resistance seen by the source goes up. So the smaller the capacitor coupled to the base, the higher its resistance, meaning -3dB roll off is at a higher frequency. You adjust this high frequency roll off by choosing the appropriate capacitor with respect to the input resistance of the amplifier.

    So what have we done here with this bandpass filter constructed of a single transistor? We have created a bandpass filter by the choice of capacitance of two critical capacitors. We have not thrown any parts away, nor have we added any. The cost is trivial.

    But if you were to make a low pass filter, you would need at least a resistor and a capacitor or inductor, and to make a high pass filter you would need at least a resistor and a capacitor or inductor. and this is a passive circuit; there is no gain, and in fact there is a loss of signal. Therefore, go with the active BP filter.

    Since you are a beginner I'll go on with this. An inductor behaves exactly opposite with respect to frequency as capacitor does. With an inductor the reactance (resistance) goes up with frequency. So when you apply the same rules of resistors in combinations of series or parallel resistances to inductors, then the change of frequency creates a change in reactance and changes the current flow accordingly. This is a beautiful thing. We can control it and therefore make use of it.

    I encourage you to get a handle on this concept. It's a fundamental of understanding electronic circuits.
    Last edited: Jan 14, 2015
  8. MrAl

    Distinguished Member

    Jun 17, 2014


    First, are you talking about passive filters or active filters?

    For passive filters there is another problem that comes up which doesnt usually come up with active filters, and that is interaction between the two stages due to the impedance 'loading' effect of the second stage, and the output impedance of the first stage tends to detune the second stage.
    For example, if you design a high pass and low pass RC filter they might have certain cutoff frequencies, but when you connect them together they affect each other so these frequencies can change by quite a bit.

    In general when you change the gain of a filter you dont change the frequency characteristics. That's because every signal that goes into the filter gets amplified by the same amount. It's only when you can change the gain of the PASSBAND alone that you can boost the signals of interest alone. Increasing the overall gain however increases the level of all the signals.
  9. patentxnewb

    Thread Starter New Member

    Jan 14, 2015
    I'm not asking for people to do me homework assignment. I've already completed most of it, just don't know how to answer these last 3-questions.
    For those that have positively contributed, thank you very much. Although I'm still somewhat confused with the answer, I'll try my best to make sense of them.
    In reality, this is my first circuits class and I'm having a hard time understanding active bandpass filters, and what would happen if it was replaced with the combination of a HPF and a LPF. For example:

    I have a first circuit that is connected to second circuit.
    The output of the first circuit is used as an input for two filters; one being a LPF and the other being a bandpass filter.
    Now, someone is saying that the bandpass filter can be replaced with the combination of a HPF and a LPF.
    But my question is, why would someone do that? And by doing so, what consequences does it have?
  10. Veracohr

    Well-Known Member

    Jan 3, 2011
    If the bandwidth is wide it doesn't have much consequence at all, but generally bandpass filters can achieve higher Q (narrower bandwidth) than cascading high- and low-pass filters. I'd say the answer to question 1 is "depends on the requirements".