Multivibrator with wide frequency range
This circuit can produce narrow negative pulses in wide frequency range - from 50Hz to 40kHz. The output frequency depends on the values of R2, C1 - this is the frequency-determining network. If R2 = 5K, the output frequency F = 40 kHz, and if R2 = 5Meg, F = 50 Hz (with C1 = 50nF). With the R2 = 10K and C1 = 50nF circuit produces 20 kHz at its output. The frequency dependence of R2C1 is nonlinear.
Fig. 1. RC multivibrator circuit
How it works. Transistors VT1, VT2 and resistors R3, R4 work as analog of unijunction transistor. Capacitor C1 charges through the resistor R2 to the power supply voltage +9V. When the voltage across C1 reaches some upper peak value, the transistors fires. The capacitor starts discharging through resistor R2 and transistors VT1, VT2. The voltage across capacitor C1 falls down, when it get some lower peak value, the transistors get turn off. The discharge occurs fast, so the circuit produces a short pulse (see Fig. 2.). The capacitor C1 starts charging again, and so on.
Fig. 2. Plots of the output of the multivibrator with different values of R2.
The frequency range of this circuit can be change by using another capacitor C1 with other capacitance. The upper frequency is limited by parameters of transistors VT1, VT2, it can reach more than 1000 kHz.
LTSpice model of this circuit can be downloaded here: multivibrator_with_wide_frequency_range.asc