Emitter follower

Every current-mode control application that exceeds 50 percent duty cycle must have slope compensation on the current ramp waveform. Otherwise an instability will occur whenever the duty cycle exceeds 50 percent. This is typically done by summing into the current waveform some of the oscillator ramp waveform. This will increase the slope of the current waveform and therefore trip the current sense comparator earlier. A common problem is the inadvertent loading of the oscillator, so I will use a PNP emitter-follower to buffer the oscillator. The circuit configuration can be seen in Figure 3-74. 

The components connected between the emitter-follower and the currentsensing filter capacitor can be thought of as a resistor divider. An additional 0.17 V needs to appear at pin 7 (through a 1K resistor) so the amount of current that must be contributed to that node is 0.17 V/1K which is 170 pA. The capacitive coupling of the PNP to pin 7 essentially centers the oscillator waveform upon the current ramp. So,... 

EXERCISE 5-12 Find the output impedance of the emitter-follower ... 

Fig. 8-3 Schematic drawing of a temperature jump apparatus. (A) Light source (B) monochromator (C) observation cell (D) photomultiplier, emitter follower (E) oscilloscope (F) spark gap (G) high voltage.

Since the voltage gain for the common-coUector stage is positive and usually close to unity, the AC voltage at the emitter will rather closely follow the voltage at the base, hence, the name emitter-follower that is usually used to describe this circuit. 

The oscillator s output is usually taken from the collector, but the buffer s output can be either from its collector (a "collector follower, CF") or its emitter (an "emitter follower, EF"). If a low impedance device such as a loudspeaker is to be driven, the emitter follower (EF) can be used, with no collector resistor. That would be a common collector buffer, because both the oscillator that is the "driver" of the buffer, and the load might be attached directly to the same "common" +9 volt "bus" or "rail" (see index if necessary). 

The op-amp connection to make a follower circuit (emitter follower, etc.) is shown in Figure 7.23. 

The analog signal is applied at the inverting terminal of the timing comparator All through emitter follower Q13 Qlb works as a 10mA current source. The circuit up to the All Inputs, when the crossover mode is selected, is shown in Fig. 7... 

The circuit diagram of this probe is illustrated in Fig. 11.11. The high voltage DC/DC converter is on the upper half of the drawing. An audio frequency oscillator composed of three NOR circuits drives the base of Q102. The step-up transformer T1 is in the collector circuit of Q102, which is fed by the emitter follower Q101. 

On the secondary side of the step-up transformer a ten-stage voltage multiplier operates delivering approximately + 1000 volts, if 10 volts are present on the base of the QlOl emitter follower. 

Photoartivated localization microscopy (PALM), fluorescence photoartivation localization microscopy (FPALM), and stochastic optical reconstruction microscopy (STORM) are the abbreviations under which this concept is commonly known at present and they all rely on stochastic on/off switching and localization of individual emitters, followed by image reconstruction. 

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