Could someone confirm my conversion of this transformer coupled BJT amplifier into a \$r_e\$ model?


simulate this circuit – Schematic created using CircuitLab

I get \$I_E\$ = 3.11mA and \$r_e\$ = 8.04Ω. No problem there.

I'm not completely sure of how the \$r_e\$ model handles the transformer coupling. \$R_C\$ is replaced by the resolved R'.

\$R' = α^2 \ R_L = 12^2 \times 8Ω = 1152Ω\$

\$V_{out} = 1/12 V' = 0.0833 V'\$


simulate this circuit

I've searched the web for an example with no success.

  • 1
    \$\begingroup\$ looks reasonable. The "quickie" check for voltage gain from Vin to collector voltage is: gain = 1152/8.04. This gain gets divided by the 12:1 output transformer. BTW, it is a reasonable design for headphones, maybe with a bit more Ic. I've used 5mA. You can improve linearity at the expense of gain with a small R in series with 100 uF. \$\endgroup\$
    – glen_geek
    Nov 4, 2018 at 4:37
  • \$\begingroup\$ Some ampliifers have lots of thermal-distortion, and bass tones cause a modulation of the gain, that gain affecting ALL the music and the high-freq tones now are tiny AM_radio_transmitters with sidebands defined by the bass. \$\endgroup\$ Nov 4, 2018 at 4:59


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