A problem with a complementary pair of transistors

清苦微甜

A problem with a complementary pair of transistors [Copy link]

The picture shows a push-pull emitter follower in the book "Transistor Circuit Design". A complementary pair of transistors is used, and the emitter current of Tr1 is about 1mA, so why is the current of Tr3 also 1mA? Why does the author say "If the absolute values of Vbe of Tr1 and Tr3 are equal, then the currents are equal"? I simulated it and the result is correct, but I don't understand how to derive it? Please teach me, thank you!

cruelfox

If other conditions are the same (the main influencing factor is temperature), Ic is a function of Vbe. This function is controlled by the parameters of the tube in the SPICE model. The actual situation will be different. Different types of tubes will have different current Ic at the same temperature, the same Vce, and the same Vbe. The same type of tubes are not exactly the same (discreteness), and the current is also different.

清苦微甜

cruelfox posted on 2018-1-15 15:00 If other conditions are the same, Ic is a function of Vbe.

I found it, Ie = IES * exp(Vbe/Vt). Since the input and output DC potentials are complementary tubes, they are both 0V. The emitter potential of Tr1 is equal to the base potential of Tr3, so Vbe is equal. Thank you

cruelfox

The circuit in the figure has no resistor in series at the emitter of Tr3 Tr4, which will cause some problems. When the temperature rises, Ic increases at the same Vbe, so the power consumption increases, further aggravating the heat generation - and finally causing the tube to burn out.

清苦微甜

cruelfox posted on 2018-1-15 15:23 The circuit in the figure does not have a resistor in series at the emitter of Tr3 Tr4, which will cause some problems. When the temperature rises, Ic increases at the same Vbe, so the power consumption increases...

It makes sense. The tube here is always on. If the output is turned on alternately, does it mean that the resistor is not needed?

cruelfox

清苦微甜 posted on 2018-1-15 15:45 That makes sense. The tubes here are always on. If the output is alternately turned on, does it mean that the resistor is not needed?

How can push-pull tubes ensure alternating conduction? Either there is an input interval where both tubes are cut off, which results in great distortion and is only suitable for use in switching circuits.

hk6108

cruelfox posted on 2018-1-15 15:23 The circuit in the figure does not have a resistor in series at the emitter of Tr3 Tr4, which will cause some problems. When the temperature rises, Ic increases under the same Vbe, so the power consumption increases...

Pure Class A or Class AB amplifiers are often used in analog circuits. The upper and lower tubes are always directly connected, so the output power cannot be greater than Pcm. If the place allows, don't save on the heat sink. If there is no emitter resistor, add an NTC between the bases of the two tubes (Tr3, Tr4) in the next stage. This NTC must be close to the heat sink.