Amplifier and signal processing apparatus

A signal processing apparatus, comprising:

• an amplifier;
• and
• a controller that controls the amplifier,
• the amplifier including:
• a P-type transistor and an N-type transistor that are connected in series,
• an operation amplifier having an output terminal, an inverting input terminal, and a non-inverting input terminal, the output terminal being coupled to a gate side of one of the P-type transistor and the N-type transistor, one of the inverting input terminal and the non-inverting input terminal being coupled to drain sides of both of the P-type transistor and the N-type transistor, a reference voltage being applied to the other of the inverting input terminal and the non-inverting input terminal,
• a transformer having a primary coil and a secondary coil, the primary coil being coupled to a source side of one of the P-type transistor and the N-type transistor, and
• a variable attenuator provided between the secondary coil and gate terminals of both of the N-type transistor and the P-type transistor, wherein
• the controller controls the variable attenuator by a control signal corresponding to a reception level of a signal outputted from the amplifier, or a reception level of a signal generated on a basis of the signal outputted from the amplifier.