Calculating gain for 4049 inverter stages?

[soggybag]

Is there a quick and dirty calculation for gain on 4049 inverter stages?

[Nasse]

Yes, took a copypaste from my old post

Here is a formula for calculating the gain of a cmos inverter arranged as inverting amplifier stage:

A = - ((R2/R1)/(1+1/Ao+R2/AoR1))

C1, C2 = DC blocking caps, usually used at input and output circuitry (not included in the formula)
R1 = input resistor
R2 = feedback resistor
Ao = gain of open inverter stage (no negative feedback)

Ao is strongly depending supply voltage, frequecy and tolerances, and varies between manufacturers and individual chips, tolerance may be more than 20%. My data sheet suggests nominal typical values for Ao at different supply voltages, like used to be in the good old days something like 50 at 9 volts, dunno what it is really now

[anchovie]

I asked on here before about stages that don't have an input resistor and Gez explained that in this case the output impedance of the previous stage forms the input resistance.

[Nasse]

I think inverter stage does have high input impedance without input resistor... and output impedance too

[anchovie]

I ended up using the "breadboard and ears" method and didn't take any measurements so I couldn't say what gains I had in the end. Just played around with the "no input resistor" stages until it sounded best!

[StephenGiles]

"Breadboard and ears" - I love it, works every time!

[brett]

quick and dirty  formula = feedback resistor divided by input resistor  (e.g. 47k input, 470k feedback, gain = 10)

quick and dirty formula is, by definition, quick and dirty and therefore doesn't always work (e.g. 4.7 k input, 47 k feedback, gain might equal 5.  Why?  Output impedance of previous stage is probably 5k, so *total* input impedance is about 10k, feedback = 47k, gain = 5)

quick and dirty formula (i) works best with large value input resistors (more than 22k),  and (ii) indicates the maximum gain you can get in real life.

just my 2c