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Emitter Coupled Multivibrator

IP.com Disclosure Number: IPCOM000080570D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 4 page(s) / 78K

Publishing Venue

IBM

Related People

Dobson, DR: AUTHOR [+2]

Abstract

This astable emitter-coupled multivibrator utilizes a common capacitor charge-current source to provide symmetry, and an amplifier to increase the gain in the positive feedback loop and to provide a signal to switch the common charge-current source.

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Emitter Coupled Multivibrator

This astable emitter-coupled multivibrator utilizes a common capacitor charge-current source to provide symmetry, and an amplifier to increase the gain in the positive feedback loop and to provide a signal to switch the common charge-current source.

Transistors T1 and T2 together with C1 form the basis of an emitter-coupled multivibrator circuit. T20 and R3 form a constant current source which is the load resistance for T2. T3 and T4 are switching devices, to switch the common charge-current source T5 to the appropriate direction through C1.

Diodes D1 through D4 are used to clamp the peak-to-peak voltage swing of the positive feedback loop. Transistor T2 of the multivibrator is driven alternately toward saturation and into cutoff by the charging and discharging action of C1, coupled with the positive feedback voltage driving T1 alternately into its active and cutoff regions.

The amplifier prevents loading the relatively high-output impedance of the multivibrator and provides positive feedback to T1. The feedback drive has two constant-current sources, so that a controlled current is always maintained through the clamping diodes D1 through D4. The amplifier also provides the necessary drive for the two switching transistors T3 and T4, and provides the necessary off-chip driving power.

To follow the action of the multivrator, first assume that the capacitor voltage is zero (the midpoint of its operation) and that T2 is off. With T2 off, the current from the constant-current source T20 will drive the input to the amplifier, point C, high. Through the action within the amplifying section, point d is driven high, forward biasing diodes D1 and D2 and clamping the base of T1 two diode voltage drops above Vref. When this occurs, the positive voltage transition will be coupled to the emitter of T2 by the emitter-to-base junction of T1 and capacitor C1. Additionally, point E will be driven high and point F low. This will turn T3 off and diodes D12 and D13 will clamp the base of T4 positive, so that T4 will be turned on but not saturated. With this action, a current path is provided through C1 from point A to B via T1 and T5 to the constant-current source T5.

The voltage at point B linearly diminishes, while the voltage at A remains constant at one VBE below point D. When the voltage at the emitter of T2 reaches Vref less one VBE, T2 will be turned on and point C will be driven low. With point C low, the points D, E, and F will also be driven to their opposite levels. When point D goes low, the base of T1 will be clamped two diode voltage drops below Vref by diodes D3 and D4. T3 will now be on and T4 off. On this half-cycle, a current path is provided through C1 from point B to A via T2 and T3 to constant-current source T5. On this half-cycle the voltage at point A linearly diminishes, while point B remains clamped at Vref less one VBE.

When the voltage at point A reaches Vref less three diode drops. the emitter...