miller effect circuits

[Roff]

It gets tricky to calculate the gain when the following stage has negative feedback from its collector to its input. It is tricky to calculate the gain of that kind of stage too.

It also gets tricky to measure the gain of a high gain transistor stage that doesn't have negative feedback, so its output is extremely distorted and is nearly rectified.

I know. :cry:
I started to get into this, and then had second thoughts.

 

[audioguru]

It is too bad we can't get MStechca to invest $1.17CAN in an LM386 low distortion audio amp IC. He will be amazed how good it sounds compared to his nightmare of random parts.
The cost of alkaline batteries dropped a couple of days ago. Maybe he should get some to replace his dead ones.

 

[akg]

I'm real lost :?
Let's say that my RC network has a 1K resistor, and a 1 uF capacitor, and the frequency being received is 200Mhz.
How do I calculate the gain according to your math?

in a simple statement ,the voltage gain of a CE amp with voltage divider bias (as in ur case) is Rc/Re , if u parallel 1uf across Re and apply 200Mhz( :shock: ) , it is as good as short ckt , further the calculation at that freq is not as simple as the above eqn: - advice- learn high freq transistor modeling and H-parameter model .

 

[mstechca]

Then if the emitter resistor is bypassed with a capacitor, the gain becomes the collector resistor divided by the capacitor's reactance.

I thought the capacitors reactance had something to do with it.

Could I not also calculate it by taking the collector resistor divided by the result from taking the capacitors reactance and the emitter resistor in parallel?

The two answers won't be too much different, but when dealing with VHF/UHF, I want to take every number seriously.

You say that the emitter resistor is 1k but don't say the collector resistor's value. Assume it is 10k. Therefore the voltage gain of the transistor stage is 10 at low frequencies. The 1uF bypass capacitor's reactance equals the value of the 1k emitter resistor at 160Hz where the gain is 10 x 1.414= 14.14 times. The gain rises 10 times for each ten times the frequency so at 1600Hz the gain is 100. The transistor's built-in emitter resistance limits the gain to about 100 so at frequencies above 1600Hz the gain is 100.

Thanks for telling me the best way to calculate gain.

...invest $1.17CAN in an LM386 low distortion audio amp IC.

Did you say invest? :lol:
I don't need an LM386 if my circuit can pick up TV stations. I'm sure the LM386 has at least one limitation that I need to break. (like highest frequency response, etc.)

The cost of alkaline batteries dropped a couple of days ago. Maybe he should get some to replace his dead ones.

I am using a 6V power supply for my testing.

 

[audioguru]

Could I not also calculate it (the voltage gain of the transistor) by taking the collector resistor divided by the result from taking the capacitors reactance and the emitter resistor in parallel?

Yes, up to a gain of about 100 due to the transistor's internal emitter resistance) but the load for the transistor and the source impedance also affect the gain.

The two answers won't be too much different, but when dealing with VHF/UHF, I want to take every number seriously.

Gain at VHF and UHF is completely different from gain at low frequencies and is much lower.

I don't need an LM386 if my circuit can pick up TV stations. I'm sure the LM386 has at least one limitation that I need to break. (like highest frequency response, etc.)

You said that your circuit has low gain and high distortion. An LM386 can have a gain of 200 if you add a capacitor, has low distortion and its frequency response extends to 300kHz when its gain is 20.

I am using a 6V power supply for my testing.

Most audio amplifiers clip (make distortion) and are not very loud when operating from only 6V. A properly designed amp will give 1/4W into an 8 ohm speaker at clipping. With a 12V supply, the same amp will give 1.3W which is 5 times as much.

 

[hyedenny]

"poor mans varactor?" The last time I checked, varactors were only about 18 cents. Ive got a whole bag of miller capacitors Im sure you can use that I'll sell you - real cheap!