Analog Amplifier Design [need assistance for the next couple of hours]

EEngineer said:

@audioguru: That's a good question. This same setup would probably not work since its not a generic setup. For different MOSFET's, won't you just bias it accordingly?

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There is no such thing as a generic Mosfet. each one is different even if they have only one part number.
they have a range of voltages that turn them on.

I spaced them out so it can be simpler for me to visualize things.

Click to expand...

Then the circuit might oscillate at a very high frequency due to the high stray capacitance.
Many high frequency circuits do not work if they are built on a breadboard.

Many years ago I made a circuit on a breadboard. It didn't work. Then I made it on a compact stripboard and it worked perfectly. I never used a lousy old breadboard again.

audioguru said:

There is no such thing as a generic Mosfet. each one is different even if they have only one part number.
they have a range of voltages that turn them on.


Then the circuit might oscillate at a very high frequency due to the high stray capacitance.
Many high frequency circuits do not work if they are built on a breadboard.

Many years ago I made a circuit on a breadboard. It didn't work. Then I made it on a compact stripboard and it worked perfectly. I never used a lousy old breadboard again.

Click to expand...

IIRC the data sheet stated the Vgs(on) as 0.8-3.0. that's more than a 3:1 range. while a circuit using one of these FETs might work the first time, it might not work so well if the FET were replaced with one, even of the same part number, or maybe even the same date code.

i always felt a bit leery of the layout of those breadboards (IIRC, they're called "vectorboards" after the company that invented them). the power rails are on opposite sides, so any bypassing will be useless because of the inductances between the rails and any caps. the long parallel springs used for the component connections must have a large amount of stray capacitance as well. they work well for what they were primarily designed for, breadboarding with TTL and CMOS logic chips in DIP packages, but for analog, they can be very squirrely. there are better proto methods for analog. vectorboards are one case where neatness can be a bad thing. i still use them for "quickie" assembly and testing of subcircuits in some projects, but then transfer it to ground-planed perf board before actual peformance testing. i built a wein bridge oscillator on vector board, and it refused to oscillate on a vectorboard, but worked just fine once i got it on perf board and point-to-point wired it.

unclejed613 said:

IIRC the data sheet stated the Vgs(on) as 0.8-3.0. that's more than a 3:1 range.

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no.
That is when it is barely turned on with a current of only 1mA.

while a circuit using one of these FETs might work the first time, it might not work so well if the FET were replaced with one, even of the same part number, or maybe even the same date code.

Click to expand...

Yes, because their spec's have such a wide range.

i always felt a bit leery of the layout of those breadboards (IIRC, they're called "vectorboards" after the company that invented them). the power rails are on opposite sides, so any bypassing will be useless because of the inductances between the rails and any caps. the long parallel springs used for the component connections must have a large amount of stray capacitance as well. they work well for what they were primarily designed for, breadboarding with TTL and CMOS logic chips in DIP packages, but for analog, they can be very squirrely. there are better proto methods for analog. vectorboards are one case where neatness can be a bad thing. i still use them for "quickie" assembly and testing of subcircuits in some projects, but then transfer it to ground-planed perf board before actual peformance testing. i built a wein bridge oscillator on vector board, and it refused to oscillate on a vectorboard, but worked just fine once i got it on perf board and point-to-point wired it.

Click to expand...

I use Veroboard (stripboard that has parallel copper strips). The copper strips are cut with a drill bit and form half of a pcb. Then the resistors, capacitors, ICs and a few short jumper wires form the remainder of the pcb.

You say Vectorboard but you mean Breadboard.
You solder parts and wires to a Vectorboard. A Vectorboard is a perf board.
You plug in parts and wires into an intermittent breadboard.

Update 2

I tried satisfying the requirements without using the feedback resistor, since that stuff is so hard for me to understand in AC Analysis >.< (something along the lines of miller effect and a weird looking feedback pi-model which completely intimidates me), and I succeeded on MultiSim.

However, when I implemented the circuit. My gain was ~ 9 no matter how much I played around with Rd, Rs1 and Rs2 whilst trying to keep the ratio of (Rd/Rs1 +Rs2) ~ 10 and a proper unsaturated output waveform.

Any ideas?

Like all mosfets the 2N7000 has a wide range of transconductance from 100 to 320 typically to (they don't say how high) at high current. They don't say how low is the transconductance at the low current you are using.

You need additional amplification from the transistor.

EEngineer said:

Update 2

I tried satisfying the requirements without using the feedback resistor, since that stuff is so hard for me to understand in AC Analysis >.< (something along the lines of miller effect and a weird looking feedback pi-model which completely intimidates me), and I succeeded on MultiSim.

However, when I implemented the circuit. My gain was ~ 9 no matter how much I played around with Rd, Rs1 and Rs2 whilst trying to keep the ratio of (Rd/Rs1 +Rs2) ~ 10 and a proper unsaturated output waveform.

Any ideas?

Click to expand...

try reducing your total Rs. it's the ratio of Rd/Rs that is the primary determining factor of the gain, and the emitter follower has a gain of LESS THAN 1.

To increase your AC gain you need to decrease Rs1. You can maintain DC stability by keeping total Rs (Rs1 + Rs2) the same.

Try Rs1 = 200, and Rs2 = 300.