Active Tape Pass Filter / Active Band Pass Filter

For a low-pass filter, the transition band starts at 0Hz or DC and continues from the maximum transition band gain to the cutting frequency point specified at -3db. Likewise, the transition band for a high-pass filter starts at this-3dB cutting frequency and continues to infinity or maximum open loop gain for an active filter. However, the active band passing filter is a frequency selector filter circuit used in electronic systems to separate a signal at a certain frequency. It is slightly different because there are a number of signals within a certain "band" of frequencies. This band or frequency range is adjusted between two cutting or corner frequency points labeled "low frequency" (εL) and "high frequency" (εH) while weakening signals outside these two points.

The simple active tape filter can be easily made by combining a single low-pass filter with a single high-pass filter, as shown.

Active Tape Passing Filter

The cutting or corner frequency of the low-pass filter (LPF) is higher than the cutting frequency of the high passing filter (HPF). The difference between frequencies at -3dB will determine the "bandwidth" of the tape-passing filter while weakening signals outside these points. One way to make a filter that pass a very simple active tape is to connect the basic passive high and low passing filters that we have looked at before to an amplifier op-amp circuit, as shown.

Active Tape Passing Filter Circuit

Active Tape Passing Filter
Active Tape Passing Filter Circuit

Combining individual low and high-pass passive filters produces a low "Q factor" type filter circuit with a wide transition band. The first stage of the filter will be the high transition phase, which uses the capacitor to prevent any DC bias from the source. This design has the advantage of producing a relatively flat asymmetric transition band Frequency response, one half representing low transition response and the other half representing high transition response as shown.

Active Tape Passing Filter

The higher corner point (εH) and the lower corner frequency breakpoint (εL) are lower than the standard first degree. This filter circuit is calculated as before in high-pass filter circuits. Obviously, a reasonable distinction between the two breakpoints is required to avoid any interaction between the low transition and high transition stages. The amplifier also provides insulation between the two stages and defines the total voltage gain of the circuit.

Therefore, the bandwidth of the filter is the difference between these upper and lower-3dB points. For example, let's say we have a tape-passing filter with 3dB breakpoints set at 200Hz and 600hz. Then the bandwidth of the filter will be given as follows: bandwidth (BW) = 600 – 200 = 400Hz.

Normalized Frequency response and phase shift for an active tape passing filter will be as follows:

Active Band Passing Frequency Response

Active Tape Passing Filter
Active Band Passing Frequency Response

Although the above passively adjusted filter circuit will work as a tape-passing filter, the transition band (bandwidth) can be quite wide. If we want to isolate a small frequency band, that could be a problem. The active tape filter can also be done using a reverse amplifier.

Therefore, by rearranging the position of resistors and capacitors within the filter, we can produce a much better filter circuit, as shown below. For a filter that pass an active band, the lower cut-off-3db point is given by εC1, while the upper cut-off-3dB point is given by εC2.

Reverse Band Passing Filter Circuit

Active Tape Passing Filter
Reverse Band Passing Filter Circuit
Active Tape Passing Filter

This type of tape-passing filter is designed to have a much narrower transition band. The central frequency and bandwidth of the filter relate to the values R1, R2, C1 and C2. The filter is also output from the output of the op-amp.

Active Filter That Undergoes Multiple Feedback Bands

We can increase the tape-passing response of the above circuit by rearranging the components to produce infinitely gaining multiple feedback (IGMF) tape-passing filters. This type of active tape transition design produces an "tuned" circuit based on the negative feedback active filter. This provides a high "Q factor" (up to 25) amplitude response and a steep rolling on both sides of the central frequency. Since the frequency response of the circuit is similar to a resonance circuit, this central frequency is called resonance frequency (εr).

Infinite Gain Multiple Feedback Active Filter

Active Tape Passing Filter
Infinite Gain Multiple Feedback Active Filter
Active Tape Passing Filter

Then we can see that the resistances determine the "Q factor" band transition of the relationship between R1 and R2 and the frequency at which the maximum amplitude occurs, the gain of the circuit will equal -2Q2. Then, as earnings increase, so do selectivity. In other words, high earnings – high selectivity..

Active Tape Pass Filter Example

One (1) Active tape passing filter with av voltage gain and 1khz resonance frequency εr is created using an infinitely gained multiple feedback filter circuit. Calculate the values of the components required to implement the circuit.

First, using the gain of the circuit to find Q, we can determine the values of the two resistances, R1 and R2 required for the active filter, as follows.

Active Tape Passing Filter

Then we can see that the value Q = 0.7071 gives a resistance relationship, R2 is twice that of resistance R1. Then we can choose any appropriate resistance value to give the necessary two ratios. Then resistance R1 = 10kΩ and R2 = 20kΩ.

The center or resonance frequency is given as 1 kHz. Using the resulting new resistance values, we can determine the value of the necessary capacitors, assuming that it is C = C1 = C2.

Active Tape Passing Filter

Resonance Frequency Point

The actual shape of the Frequency response curve for any passive or active band passing filter will depend on the characteristics of the filter circuit. The above curve will be defined as a reaction that undergoes an "ideal" band. Since there are "two" reactive components in the circuit design (two capacitors), the active band permeable filter is of type 2.

As a result of these two reactive components, the filter "Central frequency" will have a peak response or resonance frequency (εr) at εc. The central frequency is usually calculated as the geometric mean of the two-3dB frequency between the upper and lower breakpoints, which are given as the resonance frequency (oscillation point):

Active Tape Passing Filter