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Spehro 'speff' Pefhany
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The output impedance of an emitter (or source) follower is not the load resistor, as it would (approximately) be in a common emitter circuit. In fact, it is much lower- roughly the impedance the base sees divided by (hfe + 1), in parallel with the load resistor.

To see this intuitively, think of the large signal case with 10V on the collector, a stiff 5.6V on the base, and 470 ohm load. The emitter voltage is 5.0V. Now put a 470 ohm load on the emitter to ground- does the voltage drop to 2.5V as would be expected with a 470 ohm output impedance? No, it hardly drops at all- which tells you the impedance is very low.

Naturally, there must be a series capacitor or the DC emitter bias will cause problem with the input transformer or whatever it is connected to, but that's true with or without the 50 ohm resistor.

The output impedance of an emitter (or source) follower is not the load resistor, as it would (approximately) be in a common emitter circuit. In fact, it is much lower- roughly the impedance the base sees divided by (hfe + 1), in parallel with the load resistor.

The output impedance of an emitter (or source) follower is not the load resistor, as it would (approximately) be in a common emitter circuit. In fact, it is much lower- roughly the impedance the base sees divided by (hfe + 1), in parallel with the load resistor.

To see this intuitively, think of the large signal case with 10V on the collector, a stiff 5.6V on the base, and 470 ohm load. The emitter voltage is 5.0V. Now put a 470 ohm load on the emitter to ground- does the voltage drop to 2.5V as would be expected with a 470 ohm output impedance? No, it hardly drops at all- which tells you the impedance is very low.

Naturally, there must be a series capacitor or the DC emitter bias will cause problem with the input transformer or whatever it is connected to, but that's true with or without the 50 ohm resistor.

Source Link
Spehro 'speff' Pefhany
  • 422.9k
  • 23
  • 352
  • 952

The output impedance of an emitter (or source) follower is not the load resistor, as it would (approximately) be in a common emitter circuit. In fact, it is much lower- roughly the impedance the base sees divided by (hfe + 1), in parallel with the load resistor.