basic transistor amplifier

[ykurban]

How do you calculate gain of this simple amp?

[schmitt trigger]

Are you familiar with a bipolar transistor's h-parameters?
Since most resistors are well bypassed with capacitors, at a mid-frequency the circuit is reduced to a grounded emitter with a single collector resistor load.

Although you can calculate the gain of the stage by itself, for the actual gain you require to know the load impedance and the source impedance.

[graybeard]

If you can assume beta is large then the base voltage is:

VB = R8 / (R8 + R9 + R10 + R11) * 9V = 2.96V

The emitter voltage is:

VE = VB - VBE = 2.96 - 0.68 = 2.28V

The emitter current is therefore:

IE = VE / R12 = 84µA

since we assumed beta is large:

IC = IE

the transistor transconductane is:

gm = IC / VT = 3.36 mS

where VT = k * TK / q = 25mV

the mid band voltage gain from the base to the collector is:

AV = gm * R4 = 3.36ms * 2.2KOhms = 6.72

As schmitt trigger said the load and source impedances will alter this result.

[duak]

Graybeard, I remember doing those calculations in college and was able to determine the DC conditions.  I remember the next steps used IE and the 25 (or 26 mV) constant but not that it gave the transconductance.  Use it or lose it, indeed.

[graybeard]

Graybeard, I remember doing those calculations in college and was able to determine the DC conditions.  I remember the next steps used IE and the 25 (or 26 mV) constant but not that it gave the transconductance.  Use it or lose it, indeed.

In my case it has been more than 30 years since I taught this as graduate assistant, but I still use it for work.

[bson]

the mid band voltage gain from the base to the collector is:

AV = gm * R4 = 33.6ms * 2.2KOhms = 67.2

Shouldn't this be gm * (R4 + R5 + R6)?

Edit: oh wait, nvm, I wasn't awake enough to note it's an AC property.  R4 alone leads to ground.

[xavier60]

AV = 67.2 seems high. At college we were taught Re = 29/I, I in milliamps. We weren't told were the 29 came from.
Re = 29/0.084mA = 345Ω
 So I get an AV of 6.37.

[graybeard]

AV = 67.2 seems high. At college we were taught Re = 29/I, I in milliamps. We weren't told were the 29 came from.
Re = 29/0.084mA = 345Ω
 So I get an AV of 6.37.

I slipped a decimal place in the quick calculations.  gm=3.36mS not 33.6mS

[vk6zgo]

I hate the complex looking bias circuit, although it is mainly just an artifact of the simulator rendition.
My first approximation for gain, just looking at the 2.2k load, was "Gain--- not very much!"

Luckily greybeard & xavier60 have "done the hard yards" so I didn't have to !
I remembered from back in the mists of time, "gm Rc" (actually, it was "gm Rp" when I first met it).

[graybeard]

(actually, it was "gm Rp" when I first met it).

Only for pentodes and perhaps tetrodes.

[basinstreetdesign]

It has also been a long while since I did this but my calc agrees with greybeard.  6.7 looks right.

[vk6zgo]

(actually, it was "gm Rp" when I first met it).

Only for pentodes and perhaps tetrodes.

Indeed, but transistors look more like pentodes than triodes.