A musical instrument preamplifier includes a n-type JFET and a pnp current mirror connected to the drain side of the JFET. The pnp current mirror includes two pnp transistors. The current mirror is configured to control the current to independently set the operating point of the JFET and the output. An npn transistor is connected to one of the pnp transistors of the current mirror to form an inverted Sziklai pair. An auto-bias network is connected between the npn and pnp transistors that form the Sziklai pair.

Disclosed are a method, system and electronic device method for adaptively adjusting frequency of a switching power supply. The method includes: obtaining a SNR threshold, obtaining a SNR value of an AM/FM radio demodulator; when the SNR value is smaller than a SNR threshold, outputting a plurality of control signals to the switching power supply, obtaining a plurality of corresponding operation frequencies of the switching power supply; obtaining a plurality of SNR values of the AM/FM radio demodulator at each of the plurality of operation frequencies, obtaining a maximum SNR value among the plurality of SNR values; outputting a control signal corresponding to the maximum SNR value, controlling the operation frequency of the switching power supply with the control signal; obtaining the SNR value of the AM/FM radio demodulator at a preset time period, and comparing the SNR value to the SNR threshold.

In an example, a circuit includes a first field-effect transistor (FET) having a gate and first and second terminals. The circuit includes a second FET having a gate and first and second terminals, the second terminals of the first and second FETs coupled together. The circuit includes a first boosted follower coupled to the gate of the first FET and includes a second boosted follower coupled to the gate of the second FET. A third FET is coupled to the first boosted follower and the second voltage terminal and configured to turn off the first boosted follower responsive to a first level of an output voltage. A fourth FET is coupled to the second boosted follower and the first voltage terminal and configured to turn off the second boosted follower responsive to a second level of the output voltage.