Clipper and clipper using Operational Amplifier

Clipper and clipper

Applications:

Wave shaping circuits are commonly used in digital computers and communication such as TV and FM receiver.

Wave shaping technique include clipping and clamping.

In op-amp clipper circuits a rectifier diode may be used to clip off a certain portion of the input signal to obtain a desired o/p waveform.

The diode works as an ideal diode (switch) because when on, the voltage drop across the diode is divided by the open loop gain of the op-amp. When off (reverse biased) the diode is an open circuit. In an op-amp clamper circuits, however a predetermined dc level is deliberately inserted in the o/p volt. For this reason, the clamper is sometimes called a dc inverter.

Positive and Negative

Clipper: Positive Clipper:

A circuit that removes positive parts of the input signal can be formed by using an op-amp with a rectifier diode. T he clipping level is determined by the reference voltage V_ref, which should less than the i/p range of the op-amp (V_ref < V_in). The Output voltage has the portions of the positive half cycles above Vref clipped off.

The circuit works as follows:

During the positive half cycle of the input, the diode D₁ conducts only until V_in = V_ref. This happens because when V_in <V_ref, the output volts V₀ of the op-amp becomes negative to device D₁ into conduction when D₁ conducts it closes feedback loop and op-amp operates as a voltage follower. (i.e.) Output V0 follows input until V_in = V_ref.

When V_in > V_ref => the V₀ becomes +ve to derive D₁ into off. It opens the feedback loop and op- amp operates open loop. When V_in drops below V_ref (V_in<V_ref) the o/p of the op-amp V₀ again becomes –ve to device D₁ into conduction. It closes the feedback path. (o/p follows the i/p).

Thus diode D₁ is on for v_in<V_ref (o/p follows the i/p) and D₁ is off for V_in>V_ref.

The op-amp alternates between open loop (off) and closed loop operation as the D₁ is turned off and on respectively. For this reason the op-amp used must be high speed and preferably compensated for unity gain.

Ex: for high speed op-amp HA 2500, LM310, μA 318. In addition the difference input voltage (Vid=high) is high during the time when the feedback loop is open (D₁ is off) hence an op-amp with a high difference input voltage is necessary to prevent input breakdown. If Rp (pot) is connected to –VEE instead of +Vcc, the ref voltage Vref will be negative (Vref = -ve). This will cause the entire o/p waveform above –Vref to be clipped off.

Negative Clipper:

The positive clipper is converted into a –ve clipper by simply reversing diode D₁ and changing the polarity of V_ref voltage. The negative clipper clips off the –ve parts of the input signal below the reference voltage. Diode D₁ conducts -> when V_in > -V_ref and therefore during this period o/p volt V0 follows the i/p volt Vin. The –Ve portion of the output volt below –Vref is clipped off because (D₁ is off) V_in<-V_ref. If –V_ref is changed to –V_ref by connecting the potentiometer R_p to the +V_cc, the V₀ below +V_ref will be clipped off. The diode D1 must be on for V_in > V_ref and off for V_in.

Positive and Negative Clampers:

In clamper circuits a predetermined dc level is added to the output voltage. (or) The output is clamped to a desired dc level.

1.     If the clamped dc level is +ve, the clamper is positive clamper

2.     If the clamped dc level is –ve, the clamper is negative clamper.

Other equivalent terms used for clamper are dc inserter or restorer. Inverting and Non-Inverting that uses this technique.

Capacitor:

The Value of the capacitors in these circuits depends on different input rates and pulse widths.

1. In both circuits the dc level added to the o/p voltage is approximately equal to V_cc/2.

2. This +ve fixed dc level is needed to obtain a maximum undistorted symmetrical sine wave.

Peak clamper circuit:

In this circuit, the input waveform peak is clamped at Vref. For this reason, the circuit is called the peak clamper.

First consider the input voltage Vref at the (+) input: since this volt is +ve, V‘0 is also +ve which forward biases D₁. This closed the feedback loop.

Voltage V_in at the (-) input: During its –ve half cycle, diode D₁ conducts, charging c; to the –ve peak value of V_p. During the +ve half cycle, diode D1 in reverse biased. Since this voltage Vp is in series with the +ve peak volt Vp the o/p volt V0 = 2 Vp. Thus the nett o/p is Vref plus 2 Vp. So the – ve peak of 2 Vp is at Vref. For precision clamping, C_iR_d << T/2

Where R_d = resistance of diode D₁ when it is forward biased.

T = time period of the input waveform.

Resistor R is used to protect the op-amp against excessive discharge currents from capacitor Ci especially when the dc supply voltages are switched off. A +ve peak clamping is accomplished by reversing D1 and using –ve reference voltage (-V_ref).

Note:

Inv and Non-Inv clamper – Fixed dc level

Peak clamper – Variable dc level