# Small voltage amplifier design help

#### outerLimit55

Joined Mar 31, 2024
3
Any ideas for this:

Design a small-signal voltage amplifier using your choice of 2N3904/3906 bipolar junction transistors and/or 2N3819 nchannel MOSFETs that achieves the following specifications from an Rsource of 1 kΩ into an output load of 5 kΩ in parallel with 1 nF:

• Rin ≥ 1 MΩ
• Rout < 100 Ω
• AV = 12 dB, stabilized with negative feedback
• Vout > 18 Vp-p
• Bandwidth: at least 100 kHz

Choose an appropriate amplifier topology and be sure to include the following:
1) Load-line analysis to select a power supply voltage, DC operating point, and drain/collector load
2) DC bias network design
3) Open-loop small-signal analysis (midband gain, input and output impedances, high- and low-frequency response)
4) Feedback network design
5) Closed-loop small-signal analysis
6) Simulation in LTSPICE to verify design requirements have been met (AC and transient)

#### outerLimit55

Joined Mar 31, 2024
3
Thanks

#### Ian0

Joined Aug 7, 2020
10,001
Is it homework?
If so, how would you approach it?

#### outerLimit55

Joined Mar 31, 2024
3
I was planning on using a common source jfet amplifier in series with a common emitter voltage follower. I started by designing a jfet amplifier that would get me enough gain to last through the voltage follower, and correct anymore with negative feed back. Any advice on where to start with jfet amplifier would be appreciated, and if my approach is terrible, any suggestions on a better way would also be appreciated.

#### Ian0

Joined Aug 7, 2020
10,001
As I don't have to do homework any more, I'd have simply used an op-amp.
JFET is a good start, because you can easily achieve the 1MΩ input impedance.
The first thing that stuck me about the problem is the size of the input signal - not exactly small in my opinion.
I'll let you calculate it!
Does it have a low-frequency cut-off? It would be trickier if it had to work all the way down to DC, and it doesn't say in the spec.

#### WBahn

Joined Mar 31, 2012
30,236
"Small signal" doesn't require that the signal be "small" in any absolute sense, but rather that it just be "small enough" that circuit behaves sufficiently linear over that range of signals.

#### Ian0

Joined Aug 7, 2020
10,001
"Small signal" doesn't require that the signal be "small" in any absolute sense, but rather that it just be "small enough" that circuit behaves sufficiently linear over that range of signals.
That's exactly what I meant. With a signal of 18Vp-p -12dB, it would be all too easy to get a JFET forward biassed in a simple common source circuit.

#### WBahn

Joined Mar 31, 2012
30,236
That's exactly what I meant. With a signal of 18Vp-p -12dB, it would be all too easy to get a JFET forward biassed in a simple common source circuit.
Notice that the specs don't give the supply voltages. I once gave a design project that used ±30 V supplies with an output spec was that it be able to deliver 5 W to an 8 Ω load. In part, this was because that's what the lab had in the way of supplies that could deliver 5 W, and part of it was (admittedly with malicious intent aforethought) that it would force students to deal with thermal management (with the expected result of lots of smoked transistors and the unexpected result of one part number branding into one thumb).