Emitter-coupled multivibrator

Due to the elimination of the storage times, considerably higher frequencies can be achieved with emitter-coupled multivibrators than using saturated transistors. A suitable circuit is shown in Fig.1.14.

To explain how the circuit operates, let us assume that the amplitude of the AC voltages present is small at all points in the circuit, say U_PP » 0.5 V. When T₁ is OFF, its collector potential is virtually equal to the supply voltage, resulting in an emitter potential of V⁺ - 1.2 V at T₂. Its emitter current is I₁ + I₂. In order to produce the required amplitude of oscillation at R₁, the value R₁ = 0.5 V/(I₁ + I₂) must therefore be selected. This gives us an emitter potential of V⁺ - 1.1 V at T₄ under this operating condition. As long as T₁ is OFF, the current from the left current source flows via capacitor C, causing the emitter potential of T₁ to fall at a rate of

.

T₁ begins to conduct when its emitter potential has fallen to V⁺ - 1.7 V. The base potential of T₂ then falls by 0.5 V, turning the device off, and its collector potential rises to V⁺. The base potential of T₁, rises with it via emitter follower T₄, causing the emitter potential of T₁ to jump to V⁺ - 1.2 V. This step is transferred via capacitor C to the emitter of T₂, producing a potential increase from V⁺ - 1.2 V to V⁺ - 0.7 V across this device.

Date: 2015-01-12; view: 2120