Clamp amplifier circuit diagram

As shown in the figure, it is a clamp amplifier, which is easy to connect to other circuits. The most basic circuit in broadcasting or distribution of various analog video signals uses AC coupling. For this method, a clamp amplifier is first needed to reproduce the DC signal. For traditional clamp amplifiers, the input terminal adopts a capacitive coupling method. In this method, the capacitor is forced to charge and discharge during the horizontal blanking period to adjust the DC level, but the charge and discharge current of the capacitor will flow through the signal source in the reverse direction. When the usual clamp amplifier is used alone, the video input terminal is connected to a 75Ω terminal resistor, so there is not a big problem. However, for the same input signal, when other clamp amplifiers and synchronous separation circuits are connected, the charge and discharge current through the input end will cause interference, thus affecting the normal operation of the circuit.

In the figure, EL4093C is used to solve these problems. EL4093C is a clamp amplifier integrated circuit composed of an offset current added to the on-chip input amplifier. At this time, there will be no mutual interference. The input and output of the video signal is a current feedback operational amplifier A1 with a gain of 2 times, but the inverting input terminal (pin 16) of A1 is connected to the current source terminal (pin 2) that provides offset. The current value is determined by the pin connected to pin 7. The voltage of the holding capacitor C is controlled. A2 is the error amplifier. The video output signal is added to the non-inverting input terminal of A2, and the black level voltage is added to the inverting input terminal of A2. During the clamping period, the voltage on C1 is controlled so that the two voltages are equal.

The resistance of the feedback resistor R2 determines the frequency characteristics and the working stability of the circuit and cannot be selected arbitrarily. Select C1 by considering the tracking and attenuation properties from the perspective of clamping pulse width and horizontal scanning frequency. A smaller capacity is enough. It is recommended to use ceramic or polypropylene capacitors. C2 and R3 form a filter circuit to prevent noise from being mixed in.