On Tue, 17 Mar 1998 12:41:19 +1200 Steve Baldwin writes: Horowitz and Hill have an example, >but >> with >> a miniature incandescent lamp that I can't identify. Adjustable >frequency >> would >> be a special bonus. > >The lamp is used to provide automatic gain because it has a positive >temperature coefficient. ie. As it heats up, the resistance goes up. In order for any oscillator to output a low-distortion sine wave, the overall gain needs to be just enough to keep it oscillating, i.e. exactly unity. If the gain is less than unity, the oscillator won't start or keep oscillating. Too much gain, and the amplitude increases until the amplifier becomes nonlinear. At this point, its effective gain is reduced until the overall gain is unity again, but the output becomes distorted. Most oscillators operate in this mode, and produce distorted outputs. The Wein bridge design is optimized for low distortion by adding the lamp to automatically reduce the gain before the amplifier is overdriven and becomes nonlinear. A lamp is ideal for this purpose because of its thermal time constant. To a signal at the oscilator frequency (of several hundred Hz or more), it appears nearly to be a linear resistance. Thus it can be included in the oscillator loop without adding (much) distortion. However, the long term average power changes this resistance. When the average power dissipated by the lamp increases, it heats up and its resistance increases. So, before the oscillator starts, the lamp is cold, and overall loop gain is maximum. The maximum gain is designed with enough margin to ensure that oscillation starts. Once AC current starts flowing through the lamp, it heats up and increases in resistance, causing the overall loop gain to decrease until it stabilizes at unity. Other circuits other than lamps have been devised to provide the automatic gain control. Most depend on the filtering action of the frequency-selective network to filter out most of the distortion they produce. The gain-reducing part is installed in the feedback path between the amplifier (oscillator) output and the frequency-selective network. Often FETs are used. They are driven by an amplitude detector to produce the desired regulation effect. A lamp is nice because it combines the detector and control functions in one part. The lamp will last practically forever since it is operated at very low current, barely enough to make it glow. > >The best idea is to build one and have a play with it. Yes, that's the best way to deal with it. Just about any low-voltage, low-current lamp should work. The circuit will also work just replacing the lamp with an adjustable resistance, but it will either not oscillate reliably or it will have a distorted output. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]