The passive filter circuit is the filter circuit consisting of passive components (resistance, capacitance, inductance). It has great defects such as small circuit gain and poor driving load. Therefore, we should learn the active filter circuit.
Application of operational amplifiers: active filter circuits
Active filters refer to the filter circuit which consists of amplifiers, resistors, and capacitors. It can be used in information processing, data transmission and suppression of interference. But this filter is mainly used in low frequency range because of the limit of band of the operational amplifier. There are two instance applications.
Example application: comparators
When the feedback resistance of operational amplifiers
is removed, or the feedback resistance tends to infinity (i.e. open loop state), it is theoretically considered that the open loop magnification of the operational amplifier is infinite (in fact, it is very large, for example, the open loop magnification of LM324 amplifier is 100dB, that is, 100 thousand times). At this time, a voltage comparator is formed and the output is either high level (V +) or low level (V - or grounded). The operational amplifier outputs low level when the positive input end voltage is higher than that of the negative input end.
In the figure, the two operational amplifiers consists of a voltage limit comparator. Resistors R1, R1' make up a bleeder circuit to set a comparative level U1 for operational amplifier A1. The resistance R2 and R2' are composed of the sub-voltage circuit, which sets the comparative level U2 for operational amplifier A2. The input voltage U1 is added to the positive input end of the A1 and the negative input of the A2 at the same time. When Ui >U1, A1 outputs the high level; when Ui >U2, if the input voltage Ui is without [U2 , U1], the LED light is on, and this is a voltage double limit indicator. If U2 > U1 is selected, the LED is on when the input voltage is in the range of [U2 , U1], which is a "window" voltage indicator. In combination with various kinds of sensors, this circuit is used for two limits detection, short circuit, circuit breaker as long as changes a little.