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I've been reading up on small signal analysis to try and better understand audio amplifier circuit design (end goal a discrete BJT guitar amplifier). I'm attempting to apply the knowledge I've picked up (from Microelectronic circuits - Sedra/Smith) to the following amplifier schematic:

enter image description here

Would anyone be able to offer some guidance on whether or not I'm doing the correct analysis?

Assumptions: $$\beta = 100$$ $$V_{A} = 100$$ $$V_{be} = 0.7$$

So: $$(1) = 7mA$$ $$(2) \approx 4.6mA$$

Signal Input Impedance (R IN1): $$r_{\pi} = \frac{\beta \cdot V_{T}}{I_{C}}$$ $$r_{\pi} = \frac{100 \cdot 25mV}{2.3mA} \approx 1087 \Omega$$ $$R_{IN1} = R9 || r_{\pi}$$ $$R_{IN1} = 100k \Omega || 1087 \Omega$$ $$R_{IN1} \approx 1075 \Omega$$

Diff Pair Signal Output Impedance (R OUT1): $$r_{o} = \frac{V_{A}}{I_{C}}$$ $$r_{o} = \frac{100V}{2.6mA}$$ $$r_{o} \approx 38462 \Omega$$ $$R_{OUT1} = r_{o}$$

Gain (Avo): $$A_{vo} = -g_{m} \cdot r_{o}$$ $$A_{vo} = - \frac{I_{C}}{V_{T}} \cdot \frac{V_{A}}{I_{C}} = \frac{V_{A}}{V_{T}}$$ $$A_{vo} = -\frac{100V}{25mA}$$ $$A_{vo} = 4,000 V/V$$

Many Thanks in advance

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  • \$\begingroup\$ Why don't you plug the circuit into a simulator and find out yourself? You'll get a much more accurate picture than trying to resolve things with overly-simple maths and random comments from anyone not using a sim. \$\endgroup\$ – Andy aka Aug 14 at 7:46
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    \$\begingroup\$ Cdevelop, perhaps redraw the schematic (it's not bad, but the sections can be better isolated.) Then identify each section and add a description for them -- tell us what they do and why they are there. At that point, ask one (maybe two) questions that are more important to you about a section, where the nicely organized and section-isolated schematic provides the context for the question on a section. For example, it takes a moment to see what the VBE-multiplier separates and it doesn't deal with the Early Effect. It has a pot, but why between base and emitter and not base and collector? \$\endgroup\$ – jonk Aug 14 at 7:58
  • \$\begingroup\$ Thanks @jonk I'll do just that - appreciate the guidance again! \$\endgroup\$ – Cdevelop Aug 15 at 11:04

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