Technique for duty cycle shift in hard disk drive write system

A circuit for providing a write current having a programmably adjustable duty cycle in a hard disk drive write channel, comprising:

• an input differential pair gain circuit for receiving a data input signal and producing an input circuit differential output voltage;
• a differential pair emitter follower circuit receiving the input circuit differential output voltage and providing an emitter follower circuit differential output voltage;
• an output differential pair gain circuit receiving the emitter follower circuit differential output voltage and producing an output circuit differential output voltage for generating write head currents;
• and first and second programmable current sources connected to the input differential pair gain circuit to create a voltage offset of the input circuit differential output voltage.

A write data signal circuit for a hard disk drive write head, comprising:

• an output gain stage, comprising a first differential pair of bipolar junction transistors to provide write currents to the write head;
• an emitter follower stage, comprising a second differential pair of bipolar junction transistors for providing drive signals to the output gain stage;
• an input gain stage, comprising a third differential pair of bipolar junction transistors and resistive loads for the third differential pair of bipolar junction transistors for providing drive signals to the emitter follower stage;
• and first and second programmable current sources respectively connected between the third differential pair of bipolar junction transistors the resistive loads whereby changes in current supplied by the first and second programmable current sources changes a duty cycle of the write currents supplied to the write head.