PART12Pulse Circuit

Experiment Purpose

• 1.Investigate characteristics of clipping circuit.
• 2.Investigate characteristics of Schmitt-trigger circuit through experiment.
• 3.Investigate characteristics of bistable multivibrator circuit experiment.
• 4.Investigate characteristics of monostable multivibrator circuit through experiment.

The circuit used to transfer certain parts of wave form, which are higher or lower than certain standard level, is called clipping circuit, limiter or amplitude limiter.

Experiment 1 :Clipping Circuit

Theory

In fig.12-2(b), if partial linear model of the diode is used, the transfer characteristic of fig.12-2(a) can be earned. Compared to the threshold voltage(or block offset) voltage V_r, when the input voltage ν_i is ν_i≤V_r+V_R, the diode voltage becomes as ν<V_r and the diode is cutoff. Here, the electric current does not flow to the resistance R so the output ν0 becomes ν₀=ν_i. When the input signal is within the range from (-) value to V_R+V_r, the transfer characteristic curve becomes a straight line whose gradient is 1. Next, if the input signal ν_i becomes bigger as ν_i≥V_R+V_r, D is conducted and operated as a battery V_r whose inner resistance is R_f. Here, the output voltage ν₀ is as below.

That is, the increment of input △ν_i decreases, and that of output △ν₀ is as below.

You can see that this is the straight line part of the gradient R_f/(R_f + R) to ν_i > V_R + V_r in the transfer curve. The amplitude of input signal in fig.12-2(a) is a much bigger wave form than point of inflexion, so parts of (+) in the amplitude of output signal is cut. This means the cut of the input signal’s (+) maximum value, and if R_f <R, the degree of cut is much bigger so the maximum value of output signal is limited as V_R + V_r. Normally, V_R ≫ V_r, so V_R only can be used as standard voltage.

Contrary to the previous case, a diode is connected to the clipping circuit of fig.12-3(b). When partial linear model is applied to the transfer characteristic, the characteristic in fig.12-3(a) can be earned. In this circuit, the linear parts where the input signal is (+) which is bigger than V_R - V_r is transferred without attenuation, but the parts lower than (+) voltage are not output. In fig.12-2(1) and fig.12-3(a), the reverse resistance of diode R_r is supposed as R_r≫R. The circuit which cannot satisfy this condition should correct the transfer characteristic. The straight line parts whose gradient is 1 should be substituted for the gradient of R_r/(R_r+R), and in the transfer of diode clipping circuit, R_r≫R. Fig.12-4 shows a generally used diode clipping circuit. To simplify the operation of this circuit, the diode is considered ideal and the inner resistance is ignored. That is, the diode is supposed to operate as an ideal ON-OFF switch.

In the circuit of fig.12-4 (a), when the input voltage ν_i is smaller than the standard voltage V_R, D is in cutoff state, that is, the circuit is same as the open circuit, and if the input voltage ν_i is bigger than V_R, the diode is conducted, and actually operated as a short circuit. Therefore, the output V₀ is as below.

The case where the input signal is applied as sine wave is indicated in fig.12-4. The clipping wave form of the formula above is the wave form that cuts off the parts over the standard voltage V_R from input wave form and is indicated in the output side. Fig.12-4(c) is the circuit in which the polarity of D is inverted. In this circuit, when ν_i < V_R, D is conducted, and when ν_i > V_R, it is cutoff. Therefore, the output ν₀ of this case is as below.

As seen in the fig., the output wave form is the shape that the parts below the standard voltage V_R is cutoff from the input wave form. Fig.12-4(a) and (c) are called parallel clipping circuits. And fig.12-4(b) and (d) are clipping circuits composed by inputting a diode in series to the transmission line of the signal, so they are called serial clipping circuits.

Slice or Limiter Circuit

A diode clipping circuit which is composed of double diodes in order to transfer only the signals between two standard voltages V_R1 and V_R2 is called a slice(limiter) circuit. This circuit can be composed as parallel-serial or serial-parallel connection, and fig.12-5(b) shows the parallel case. The transfer characteristic curve has two points of inflexion of ν₀ = ν_i = V_R1 and ν₀ = ν_i = V_R2, and has characteristics as below. Here, V_R2 > V_R1 ≫ V_r and R_f ≪ R. When the input voltage ν_i is as below, the states of output voltage and diode are as below.

result_table

Input Signal Vi	Output Signal V0	State of Diode
Vi ≤ VR1 VR1 ≤ Vi ≤ VR2 Vi ≥ VR2	V0 = VR1 V0 = Vi V0 = VR2	D1 : ON,D2 : OFF D1 : OFF,D2 : OFF D1 : OFF,D2 : ON

This slice circuit is used to earn a square wave from a sine wave signal, and to get a symmetrical square wave, the absolute values of V_R1 and V_R2 are same but their polarities are opposite. In this condition, the transfer characteristic curve passes the origin and the input wave form cuts as its upper and lower parts becomes symmetry. When the amplitude of input signal is much bigger than the difference between the standard voltage V_R1 and V_R2, the input signal becomes a square wave.

And if zener diode is connected in series reversely as in fig.12-6(a), it is operated as a clipper which has same characteristic as fig.12-6(b).

If the diode has same characteristic, a amplification limiter which is symmetrical can be earned. If the breakdown voltage of zener diode is V_Z and the cutoff voltage is V_r, the transfer characteristic of fig.12-6(b) is earned.

Experiment Process

1. In Circuit-1 of M12, compose a circuit as in fig.12-7 and make the input as sine-wave 1000Hz.

2. As given in table 12-1, measure the output voltage between 1f-1g terminals for each input voltage and record the result in the relevant column of table 12-1. Check out the output wave form and compare it with the input wave form.

3. In Circuit-1 of M12, compose a circuit as in fig.12-8 and make the input as sine-wave 1000Hz.

4. As given in table 12-2, measure the output voltage between 1l-1m terminals for each input voltage and record the result in the relevant column of table 12-2. Check out the output wave form and compare it with the input wave form.

tab1 print