Swamped amplifier explanation

[saviourm]

Hello guys

Please I need some help regarding a swamped amplifier(circuit attached), When I started reading the circuit functional description I have been little bit confused, please see attached circuit. My question, why is there less voltage across the emitter diode during the increase of voltage of across the emitter resistor?
I think that when the voltage increases across the emitter resistor it opposes the voltage fed by the potential divider biasing of the circuit therefore less voltage across the emitter diode, but I am not certain!!! please can someone confirm my thoughts or if I am wrong a more simple explanation of the circuit?
Many thanks in advance

 

[Dan Mills]

The emitter diode voltage is trivially Vb - Ve and Ve is equal to the voltage across the emitter resistor chain, thus an increase in Ve necassarily decreases Vbe by the ame amount.

Note that this ignores both the base spreading resistance and the intrinsic emitter resistance, but those are details until you get to some better transistor models.

Regards, Dan.

 

[saviourm]

The emitter diode voltage is trivially Vb - Ve and Ve is equal to the voltage across the emitter resistor chain, thus an increase in Ve necassarily decreases Vbe by the ame amount.

Regards, Dan.

Thanks for your reply, Is Ve increase much more compererd to the increase on Vb ? to satisfy your stated formula "V emitter diode voltage= Vb - Ve"

 

[saviourm]

I am just to clear things. Is Ve increased more than Vb?so the voltage across the emitter diode(Vbe) is decrease before the last input voltage(Vb)

 

[saviourm]

please can someone help me!!!!!!!!!!!!!!

 

[godfreyl]

I am just to clear things. Is Ve increased more than Vb?so the voltage across the emitter diode(Vbe) is decrease before the last input voltage(Vb)

No. The explanation in the book is misleading / confusing.

The increase in Ve is less than the increase in Vb. For example, if you increase Vb by 50mV, Ve may increase by 45mV, so Vbe has increased by 5mV.

Where the book says "As a result, there is less voltage across the emitter diode than before", I think the author means there is less voltage across the emitter diode than before the emitter resistor was added to the circuit. or something. It really is badly worded.

 

[saviourm]

Many thanks form my heart for the help!!