Emitter-coupled Schmitt trigger

A non-inverting amplifier can also be implemented by using a differential amplifier. By applying positive feedback through a resistive voltage divider, we obtain the emitter-coupled Schmitt trigger shown in Fig.1.12. Both of its trigger thresholds are positive.

Fig. 1.12 - Emitter-coupled Schmitt trigger . Fig. 1.13 - Transfer characteristic.

By selecting suitable component values for the circuit, we can cause current I_K to switch from one transistor to the other when the circuit changes state, without the transistors becoming saturated. This eliminates the storage time t_S during switchover, and very high switching frequencies can be achieved. This principle is known as "unsaturated logic".

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.

Fig. 1.14 - Emitter-coupled multivibrator. Fig. 1.15 - Voltage waveforms.

During the OFF-time of T₂, current I₂ flows via capacitor C and causes the emitter potential of T₂ to fall at a rate of

.

Transistor T₂ remains OFF until its emitter potential has fallen from V⁺ - 0.7 V to V⁺ - 1.7 V, giving a switching time of

or, more generally, .

Similarly, we obtain

or, more generally, .

The voltage waveforms for the circuit are shown in Fig.1.15. We can see that by selecting U_PP = 0.5 V, none of the transistors is driven into saturation. This circuit allows frequencies of more than 100 MHz to be achieved with no great cost or complexity.

The circuit is particularly suitable for frequency modulation. For this purpose we select the currents I₁ = I₂ = I and control them with a common modulation voltage. In order to ensure that the oscillation amplitude at R₁ remains constant, a diode can be connected in parallel with it in each case, as shown by the dashed lines. The oscillation frequency then becomes

,

where U_D is forward voltage of the diodes.

Emitter-coupled multivibrators are available as monolithic integrated circuits, usually incorporating a TTL- or ECL-compatible output stage.

IC types:

TTL XR2209; f_max = 1 MHz (Exar)

TTL SN74LS624...629; f_max = 20 MHz (Texas Inst.)

ECL MC 1658; f_max = 150 MHz (Motorola)