All FET, SE, Class-A Headphone Amplifier
The circuit above is as about straight forward as can be imagined. The first FET operates in a grounded-source configuration and its output is directly cascaded into the second FET’s grid, which is configured as a source follower. The 10k resistor that spans the output to first FET’s grid sets the feedback path. This circuit is limited in several ways, as the output voltage swing is compromised by the source follower’s resistive loading and the fist stage’s gain is relatively small because of its drain resistor’s low value. A better circuit requires a bit more complexity. Shown below is an improved version of the circuit above. (Download circuit: version 4.2 SE 9V 2N4393 HP amp.ckt ; version 4.0 SE 9V 2N4393 HP amp.ckt)
In this remade version, the drain resistor and source follower’s load resistor have been replaced by constant-current sources. Now, the fist stage can develop much more gain and the second stage can swing much larger current swings into 32-ohm loads. (The first stage’s constant-current source may seems too elaborate, but it isn’t. If truly identical FETs could be found and glued together to ensure a constant temperature tracking, then the internal coupling capacitor and its accompanying two high-valued resistors might be eliminated. But such a tight match is difficult to find, so the need for the extra components remains, as they keep the first stage’s output centered at half the battery voltage.) Well, how good is this circuit? The frequency response extends from 7Hz to 7MHz, which should be sufficient, save for all but the most demanding audiophiles.
 |
 |
 |