Negative feedback transistor

Current limiting A technique used to provide short-circuit protection. Typically a current sensing resistor is used to turn a transistor on and off that in turn is used to provide negative feedback to hold the output current at a high but safe value. 

The lesson here is that negative feedback is more effective in causing oscillation if it is delayed somewhat, depending on the type of operation desired. (This will be seen again in Chapter 16, using transistors. positive... 

Transistors are so fast that their natural delay times are in the billionths of a second, so we ordinarily use capacitive delays to produce oscillations with somewhat longer time constants. (For example, to produce a musical note that we can hear, the delay should be roughly a thousandth of second.) We can use either delayed negative feedback (bottom of next page) or fast positive feedback (top of page 182). With transistor circuitry, it is more often directly positive. 

Troubleshooters might have to search for feedback from slight inductances or capacitances, causing undesirable "parasitic" oscillations. Sometimes an internal resistance way back in the power supply gets loaded like the emitter resistor here, causing delayed fluctuations in voltage, which can feed back to a transistor and start oscillations, from either direct positive or delayed negative feedback. 

If a transistor can be arranged to have delayed negative feedback, similar to the relay buzzer example on page 138, it can oscillate continuously. This is called "astable," meaning not stable. That is, it will not remain in one state, but instead it flips back and forth between "on" and "off."... 

With a transistor, if the feedback was immediate, there would be no oscillation. Instead, the "negative" feedback would simply decrease the degree to which the transistor would turn on, so it would only go part-way on and then remain with that partial status. In fact, this does happen with certain other types of amplifier circuits (to be studied later) but not with multivibrators. However, if the feedback is delayed enough for the transistor to turn on completely before... 

Point out that VE must be close to VF, and that VG must be near OV. Besides, the current flowing across R19 ( 5 mV) splits in almost equal parts between Q7 and Q8, about 2.5 mA in each transistor. 2.5 mA will then flow R9, thus keeping VH to about 17.7V. VL must be about. 7 V more positive. If the voltages VE, VF, VG, VH, VL differ considerably from the stated values, and especially if VG is positive or negative of some volts, then the output amplifier is defective. To troubleshoot it, first of all check the output stage and determine whether or not current flows across R24 and R26. The presence of current across R24 and R26 tells you that QIO and Qll are alive. If it is so, focus your attention on Q7, Q8, Q9. Disconnect Jumper AC-AB, in which case you open the feedback loop, and you can check the behaviour of Q7, Q8 in the open-loop situation. Disconnect Q9 and make sure that the currents across Q7, Q8 are not very different from each other, or at least that none of them are at zero. In this way you check the condition of Q7, Q8. If required, replace the defective component. Check then with the transistor tester Q9, and if necessary, replace it. Then, again connect Q9 and reintroduce the Jumper. The dc condition of the output amplifier should now be correct. 

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