Pulse Circuits - Miller Sweep Generator

The transistor Miller time base generator circuit is the popular Miller integrator circuit that produces a sweep waveform. This is mostly used in horizontal deflection circuits.

Let us try to understand the construction and working of a Miller time base generator circuit.

Construction of Miller Sweep Generator

The Miller time base generator circuit consists of a switch and a timing circuit in the initial stage, whose input is taken from the Schmitt gate generator circuit. The amplifier section is the following one which has three stages, first being an emitter follower, second an amplifier and the third one is also an emitter follower.

An emitter follower circuit usually acts as a Buffer amplifier. It has a low output impedance and a high input impedance. The low output impedance lets the circuit drive a heavy load. The high input impedance keeps the circuit from not loading its previous circuit. The last emitter follower section will not load the previous amplifier section. Because of this, the amplifier gain will be high.

The capacitor C placed between the base of Q₁ and the emitter of Q₃ is the timing capacitor. The values of R and C and the variation in the voltage level of V_BB changes the sweep speed. The figure below shows the circuit of a Miller time base generator.

Operation of Miller Sweep Generator

When the output of Schmitt trigger generator is a negative pulse, the transistor Q₄ turns ON and the emitter current flows through R₁. The emitter is at negative potential and the same is applied at the cathode of the diode D, which makes it forward biased. As the capacitor C is bypassed here, it is not charged.

The application of a trigger pulse, makes the Schmitt gate output high, which in turn, turns the transistor Q₄ OFF. Now, a voltage of 10v is applied at the emitter of Q₄ that makes the current flow through R₁ which also makes the diode D reverse biased. As the transistor Q₄ is in cutoff, the capacitor C gets charged from V_BB through R and provides a rundown sweep output at the emitter of Q₃. The capacitor C discharges through D and transistor Q₄ at the end of the sweep.

Considering the effect of capacitance C₁, the slope speed or sweep speed error is given by

$$e_s = \frac{V_s}{V} \left( 1- A + \frac{R}{R_i} + \frac{C}{C_i} \right )$$

Applications

Miller sweep circuits are the most commonly used integrator circuit in many devices. It is a widely used saw tooth generator.