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)

Date: 2015-01-12; view: 1264