Amplitude-limiting circuits are useful when a signal should not exceed a predetermined maxi-mum amplitude, such as when feeding an A/D or a modulator. A clipper, which completely removes the signal above a certain level, is useful for many applications, but there are times when it is not desirable to lose information. The circuit in this figure is a voltage-controlled breakpoint amplifier. When the input signal reaches a predetermined level (the breakpoint), the amplifier gain is reduced. As both the breakpoint and the gain for signals greater than the breakpoint are voltage programmable, this circuit is useful for systems that adapt to changing signal levels. Adaptive highlight compression finds uses in CCD video cameras, which have a very large dynamic range. Although this circuit was developed for video signals, it can be used to adaptively compress any signal within the 40-MHz bandwidth of the LT1256. The LT1256 video fader is connected to mix proportional amounts of input signal and clipped signal to provide a voltage-controlled variable gain. The clipped signal is provided by a discrete circuit consisting of three transistors. Q1 acts as an emitter follower until the input voltage exceeds the voltage on the base of Q2 (the breakpoint voltage, VBP). When the input voltage is greater than VBP Q1 is off and Q2 clamps the emitters of the two transistors to VBP plus a VBE. Q3, an NPN emitter follower, buffers the output and drops the voltage (VBE); thus, the dc level of the input signal is preserved. The breakpoint voltage at the base of Q2 must remain constant when this transistor is turning on or the signal will be distorted. The LT1363 maintains a low output impedance well beyond video frequencies and makes an excellent buffer.
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