Active Band Pass Filter

Active band pass filters are simply filters constructed by using operational amplifiers as active devices configured to simulate inductors or what are known as "gyrators". Active band pass filters are used largely at audio frequencies where otherwise the size of the inductor would become prohibitive.


The are many different types of active filters including high pass, low pass, band reject and there are numerous responses including multiple feedback band pass (MFBP), dual-amplifier band pass (DABP) and, state variable bi-quad all pole circuits. Interestingly all known filter responses such as Butterworth and Chebyshev may be synthesised.

Simple Single Op Amp Band Pass Filter

This is a circuit design for a band pass filter. A band pass filter passes a range of frequencies while rejecting frequencies outside the upper and lower limits of the pass band. The range of frequencies to be passed is called the pass band and extends from a point below the center frequency to a point above the center frequency where the output voltage falls about 70% of the output voltage at the center frequency. This is the figure of the circuit.


The filter bandwidth (BW) is the difference between the upper and lower pass band frequencies. The quality factors, or Q of the filter is a measure of the distance between the upper and lower frequency points and is defined as (Center Frequency / BW) so that as the pass band gets narrower around the same center frequency, the Q factor becomes higher. For a single op-amp band pass filter with both capacitors the same value, the Q factor must be greater than the square root of half the gain, so that a gain of 98 would require a Q factor of 7 or more.

Simple Indicates Differs Input Voltage Circuit Using CA3140

This circuit is use for indicates when the input voltage differs from two defined limits, V1 and V2. The limits that can be adjusted and the circuit is designed to trigger the adjustable “window”. Supply voltage, Vcc must be higher than the highest input voltage by at least 2 volts. One application here is to monitor a 12V car battery. V1 can be set to 14V and V2 to 11V thus giving an indication of more than the cost or weak batteries. Op-amp used here is the CA3140 MOSFET. They are used to advantage because they have less output offset voltage and can switch to 0volts close. If any other use op-amps is like the LF351 or CA741 will need to have an offset null control. This is just a 10k preset reached between pins 1 and 5, the wiper connected to the negative supply op-amps or 4 pins. This is the figure of the circuit.

With this circuit the op-amp will turn on the LED if the input voltage out of limits, the two 1N4148 diodes to form an “AND”-gate at the output. Input voltage is to be monitored are fed through a series of 10k resistors on the input of both op-amps. If the input voltage is greater than the limit set by V1 it will CA3140 output swing to almost full supply voltage and LED light. Similarly, if the input voltage is less than the limit set by V2 the op-amp will swing to the Vcc and the LED light.

Simple Ultra High Input Impedance Circuit For AC Unity Gain Amplifier

This is an ultra high input impedance AC unity gain amplifier circuit. This circuit is used to get unity gain from ultra high input impedance AC. This is the figure of the circuit.

Because we can’t reduce input capacitance of this circuit, so we use the 2N4416 which has low capacitance. It is operated as a source follower with bootstrapped gate bias resistor and resistor . [Source: National Semiconductor Application Note]