Bidirectional Sampling Gates

Bidirectional gates, unlike unidirectional ones, transmit signals of both positive and negative polarities. These gates can be constructed using either transistors or diodes. From different types of circuits, let us go through a circuit made up of transistors and another made up of diodes.

Bidirectional Sampling Gates using Transistors

A basic bidirectional sampling gate consists of a transistor and three resistors. The input signal voltage V_S and the control input voltage V_C are applied through the summing resistors to the base of the transistor. The circuit diagram given below shows the bidirectional sampling gate using transistor.

The control input V_C applied here is a pulse waveform with two levels V₁ and V₂ and pulse width t_p. This pulse width decides the desired transmission interval. The gating signal allows the input to get transmitted. When the gating signal is at its lower level V₂, the transistor goes into active region. So, until the gating input is maintained at its upper level, signals of either polarity, which appear at the base of the transistor will be sampled and appear amplified at the output.

Four Diode Bidirectional Sampling Gate

Bidirectional sampling gate circuit is made using diodes also. A two diode bidirectional sampling gate is the basic one in this model. But it has few disadvantages such as

It has low gain
It is sensitive to the imbalances of control voltage
V_{n (min)} may be excessive
Diode capacitance leakage is present

A four diode bidirectional sampling gate was developed, improving these features. A two bidirectional sampling gate circuit was improved adding two more diodes and two balanced voltages +v or –v to make the circuit of a four diode bidirectional sampling gate as shown in the figure.

The control voltages V_C and –V_C reverse bias the diodes D₃ and D₄ respectively. The voltages +v and –v forward bias the diodes D₁ and D₂ respectively. The signal source is coupled to the load through the resistors R₂ and the conducting diodes D₁ and D₂. As the diodes D₃ and D₄ are reverse biased, they are open and disconnect the control signals from gate. So, an imbalance in control signals will not affect the output.

When the control voltages applied are V_n and –V_n, then the diodes D₃ and D₄ conduct. The points P₂ and P₁ are clamped to these voltages, which make the diodes D₁ and D₂ revere biased. Now, the output is zero.

During transmission, the diodes D₃ and D₄ are OFF. The gain A of the circuit is given by

$$A = \frac{R_C}{R_C + R_2} \times \frac{R_L}{R_L + (R_s/2)}$$

Hence the choice of application of control voltages enables or disables the transmission. The signals of either polarities are transmitted depending upon the gating inputs.

Applications of Sampling Gates

There are many applications of sampling gate circuits. The most common ones are as follows −

Sampling scopes
Multiplexers
Sample and hold circuits
Digital to Analog Converters
Chopped Stabilizer Amplifiers

Among the applications of sampling gate circuits, the Sampling scope circuit is prevalent. Let us try to have an idea on the block diagram of sampling scope.

Sampling Scope

In the sampling scope, the display consists of a sequence of samples of input waveform. Each of those samples are taken at a time progressively delayed with respect to some reference point in the waveform. This is the working principle of sampling scope which is shown below in the block diagram.

The ramp generator and the stair case generator generates the waveforms according to the trigger inputs applied. The comparator compares both of these signals and generates the output which is then given to the sampling gate circuit as a control signal.

As and when the control input is high the input at the sampling gate is delivered to the output and whenever the control input is low, the input is not transmitted.

While taking the samples, they are chosen at the time instants, which are progressively delayed by equal increments. The samples consist of a pulse whose duration is equal to the duration of the sampling gate control and whose amplitude is determined by the magnitude of the input signal at the sampling time. The pulse width then produced will be low.

Just like in the Pulse modulation, the signal has to be sampled and hold. But as the pulse width is low, it is amplified by an amplifier circuit so as to stretch and then given to a diode-capacitor combination circuit so as to hold the signal, to fill the interval of the next sample. The output this circuit is given to the vertical deflection plates and the output of sweep circuit is given to the horizontal deflection plates of the sampling scope to display the output waveform.