Waveform Generators

Astable Multivibrator

An astable multivibrator is also known as a free-running multivibrator. It is called free-running because it alternates between two different output voltage levels during the time it is on. The output remains at each voltage level for a definite period of time. If you looked at this output on an oscilloscope, you would see continuous square or rectangular waveforms. The astable multivibrator has two outputs, but no inputs.

An astable multivibrator, as shown in the figure above, is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off. The astable multivibrator is said to oscillate. To understand why the astable multivibrator oscillates, assume that transistor Q₁ saturates and transistor Q₂ cuts off when the circuit is energized. This situation is shown in the figure below. We assume Q₁ saturates and Q₂ is in cutoff because the circuit is symmetrical; that is, R₁ = R₄, R₂ = R₃, C₁ = C₂, and Q₁ = Q₂. It is impossible to tell which transistor will actually conduct when the circuit is energized. For this reason, either of the transistors may be assumed to conduct for circuit analysis purposes.

Essentially, all the current in the circuit flows through Q₁; Q₁ offers almost no resistance to current flow. Notice that capacitor C₁ is charging. Since Q₁ offers almost no resistance in its saturated state, the rate of charge of C₁ depends only on the time constant of R₂ and C₁ (recall that τ = RC). Notice that the right-hand side of capacitor C₁ is connected to the base of transistor Q₂, which is now at cutoff.

Let's analyze what is happening. The right-hand side of capacitor C₁ is becoming increasingly positive. If the base of Q₂ becomes sufficiently positive, Q₂ will conduct. After a certain period of time, the base of Q₂ will become sufficiently positive to cause Q₂ to change states from cutoff to conduction. The time necessary for Q₂ to become saturated is determined by the time constant R₂C₁.

The next state is shown in the figure below. The positive voltage accumulated on the right side on capacitor C₁ has caused Q₂ to conduct. Now the following sequence of events takes place almost instantaneously. Q₂ starts conducting and quickly saturates, and the voltage at output 2 changes from approximately V_CC to approximately 0 volts. This change in voltage is coupled through C₂ to the base of Q₁, forcing Q₁ to cutoff. Now Q₁ is in cutoff and Q₂ is in saturation. Notice that the figure below is the mirror image of the figure above. In the figure below the left side of capacitor C₂ becomes more positive at a rate determined by the time constant R₃C₂. As the left side of C₂ becomes more positive, the base of Q₁ also becomes more positive. When the base of Q₁ becomes positive enough to allow Q₁ to conduct, Q₁ will again go into saturation. The resulting change in voltage at output 1 will cause Q₂ to return to the cutoff state.

Look at the output waveform from transistor Q₂, as shown in the figure below. The output voltage (from either output of the multivibrator) alternates from approximately 0 volts to approximately V_CC, remaining in each state for a definite period of time. In some applications, the time period of higher voltage (V_CC) and the time period of lower voltage (0 volts) will be equal. Other applications require differing higher- and lower-voltage times.