A driving circuit to generate a driving signal for an optical modulator in response to a differential input signal having a positive phase component and a negative phase component, the driving circuit comprising:
- a first differential amplifier configured to generate a differential output signal in response to a difference between the positive phase component and the negative phase component of the differential input signal, the differential output signal having a positive phase component and a negative phase component;
- and
- a second differential amplifier configure to generate the driving signal in response to the differential output signal,
- wherein the first differential amplifier comprises:
- an emitter follower configured to generate a first differential signal by lowering voltage level of the differential input signal, the first differential signal having a positive phase component and a negative phase component;
- a delay adjustment circuit configured to generate a second differential signal by delaying the first differential signal in response to instantaneous voltage level of the first differential signal, the second differential signal having a positive phase component and a negative phase component;
- a current source configured to supply a source current;
- a differential circuit configured to divide the source current into a first current and a second current, the first current and the second current varying complementarily in response to a difference between the positive phase component of the second differential signal and the negative phase component of the second differential signal;
- a first load resistor configured to generate the positive phase component of the differential output signal based on the first current;
- and
- a second load resistor configured to generate the negative phase component of the differential output signal based on the second current,
- wherein the delay adjustment circuit comprises:
- a positive phase input node configured to receive the positive phase component of the first differential signal;
- a negative phase input node configured to receive the negative phase component of the first differential signal;
- a positive phase output node configured to output the positive phase component of the second differential signal;
- a negative phase output node configured to output the negative phase component of the second differential signal;
- a first resistor electrically connected between the positive phase input node and the positive phase output node;
- a second resistor electrically connected between the negative phase input node and the negative phase output node;
- a first capacitor;
- a second capacitor;
- and
- a third differential amplifier having a non-inverting input node, an inverting input node, a non-inverting output node, and an inverting output node, the non-inverting input node being electrically connected between the second resistor and the negative phase output node, the inverting input node being electrically connected between the first resistor and the positive phase output node, the non-inverting output node being electrically connected to the inverting input node through the second capacitor, the inverting output node being electrically connected to the non-inverting input node through the first capacitor,
- wherein the first differential signal has a voltage level including a first voltage level, a second voltage level higher than the first voltage level, a third voltage level higher than the second voltage level, and a fourth voltage level higher than the third voltage level,
- wherein the third differential amplifier operates in a non-saturated region, when the voltage level of the first differential signal is in an input voltage range, and the third differential amplifier operates in a saturated region, when the voltage level of the first differential signal is out of the input voltage range, and
- wherein the input voltage range is a voltage range from a first voltage value to a second voltage value, the first voltage value being set to be higher than the first voltage level and lower than the second voltage level, the second voltage value being set to be higher than the third voltage level and lower than the fourth voltage level.