This circuit can be used in some automatic control systems. Resistance R1 and R2 constitute the bleeder circuit to provide bias voltage U1 for the negative input end of the operational amplifier A1 as the comparison voltage reference. Being static state, capacitor C1 finishes charging, and U2, the positive input end voltage of A1, is equal to the power voltage V+, so A1 outputs high level. When the input voltage Ui changes to low power, the diode D1 is on and the capacitance C1 discharges rapidly through the D1 so that the U2 is suddenly reduced to the ground level. At this point, because of U1>U2, A1 outputs low level. When the input voltage gets high, the diode D1 is off and the power supply voltage R3 charges the capacitor C1. When the charging voltage on C1 is more than U1, U2>U1 and A1 becomes high level, thus ending a single stable trigger. Obviously, the increase of U1 or the increase of the value of R2 and C1 will extend the time of the single delay trigger; on the contrary, the time is shorter. If the diode D1 is removed, the circuit has a power delay function. When the power is just added, the U1>U2, and the operational amplifier A1 outputs low level. With the constant charging of the capacitor C1 and the increase of U2, the output of A1 becomes high level when U2>U1.
Application of operational amplifiers -- in- phase AC amplifiers
The characteristic of the in-phase AC amplifier is that the input impedance is high. The R1 and R2 consist of 1/2V+ bleeder circuit, which offset the operational amplifier through R3. The voltage magnification times Av of the circuit is also only determined by the external resistance: Av=1+Rf/R4, the circuit input resistance is R3. The resistance of the R4 ranges form thousands of ohms to tens of thousands of ohms.
Reverse ac amplifier can also be implemented.