What is an Active Filter?
What is an Active Filter?
An active filter is a type of filter that includes one or more active circuit components such as a transistor or an operational amplifier (Op-Amp). They derive their energy from an external source of energy and use it to increase or amplify the signal output.
Operational amplifiers adjust the filter’s output by narrowing or widening the bandwidth, creating a selective response to frequencies.
An Op-Amp has a high input impedance, a low output impedance and a voltage gain within its feedback loop arising from the mixture of the resistor. Active filters, when used with careful circuit design, generate excellent performance features, very good precision with a steep roll-off and low noise.
What is an Active Low Pass Filter?
An Active Low Pass Filter only allows low-frequency signals to pass while blocking high frequencies, utilizing an Op-Amp that converts high impedance inputs to low impedance outputs.
The performance of the amplifier plays a very important factor when designing an active low pass filter. There are two primary kinds of active low-pass filters namely switched capacitor type and continuous capacitor type. The filters are available from first-order until the eighth order of design.
The circuit’s frequency response mirrors that of a passive RC filter, but with a passband voltage gain that boosts the output signal’s amplitude.
The passband begins from 0Hz or DC for a low pass filter and continues at -3dB to the designated cut-off point. Signals are attenuated beyond the cut off frequency. Active low pass filters are grouped according to the order of the filter. We will discuss 1st & 2nd order active low pass filters.
A high pass filter, the opposite of a low pass filter, allows frequencies above the cutoff and blocks lower ones. Bandpass filters merge these functions to allow only specific frequency ranges.
First Order Active Low Pass Filter
A first-order active low pass filter is a simplistic filter that is composed of only one reactive component Capacitor accompanying with an active component Op-Amp. A resistor is utilized with the capacitor or inductor to form RC or RL low pass filter respectively. In a passive circuit, the output signal amplitude is smaller than the input signal amplitude.
To surmount this problem, active circuit designs were introduced. When a passive low pass filter is connected to an Op-Amp either in inverting or non-inverting condition, it gives an active low pass filter design. The connection of a simple RC circuit with a single Op-Amp is shown in the image below.
This RC circuit assists in providing a low-frequency signal to the input of the amplifier. The amplifier operates as a unity gain output buffer circuit. This circuit has added input impedance value. The Op-Amp of the circuit has a very low output impedance value, which helps in providing high stability to the filter.
When compared with passive filter, an active filter has an arrangement in which the amplifier is designed as a voltage-follower (Buffer) which gives a DC gain of one. This configuration provides excellent stability to the filter and has a high power gain. The main drawback of this filter is that it has no voltage gain above one, which can be rectified with extra circuit design.
