Broadband driver with extended linear output voltage

Abstract

Modern modulator drivers must be capable of delivering a large output voltage into a tens of ohms modulator, while minimizing the amount of distortion added by the driver. The driver should deliver the output voltage without exceeding a maximum distortion while minimizing the DC power consumption. Accordingly, a modulator driver includes a final stage amplifier with auxiliary transistors that turn on when the conventional differential pair of transistors approaches their maximum voltage of the linear region of their transfer function, thereby providing a more linear transfer function, in particular at large input voltages.

We claim: 
     
         1 . A transmitter driver for preparing input electrical signals for output to a modulator comprising:
 first and second inputs for inputting first and second input electrical signals;   a buffer stage for shifting a reference of the first and second input electrical signals;   an amplifier stage receiving the first and second input electrical signals, including:
 first and second differential pair transistors forming a differential pair connected to the buffer stage; 
 first and second outputs connected to second terminals of the first and second differential pair transistors, respectively, for outputting first and second output electrical signals; 
   first and second auxiliary transistors connected to the second terminals of the first and second differential pair transistors, wherein the first and second auxiliary transistors are capable of turning on when the first and second input electrical signals exceed a maximum input voltage of a linear region of a transfer function of the first and second differential transistors for increasing the first and second output electrical signals, thereby increasing a linear region of a transfer function of the amplifier stage.   
     
     
         2 . The transmitter driver according to  claim 1 , further comprising a first degeneration resistor extending between third terminals of the first and second differential pair transistors capable of extending a linear voltage range of the amplifier stage. 
     
     
         3 . The transmitter driver according to  claim 1 , wherein the buffer stage comprises first and second buffer transistors connected to first terminals of the first and second differential pair transistors, respectively. 
     
     
         4 . The transmitter driver according to  claim 3 , wherein the buffer stage includes third and fourth buffer transistors including first terminals connected to the first terminals of the first and second buffer transistors, respectively, and third terminals connected to the first terminals of the first and second auxiliary transistors, respectively. 
     
     
         5 . The transmitter driver according to  claim 4 , further comprising first and second shifting resistors connected to the third terminals of the third and fourth buffer transistors, respectively, for shifting voltage applied to the first terminals of the first and second auxiliary transistors, respectively, whereby the first and second auxiliary transistors turn on when the first and second differential pair transistors exceed the maximum input voltage of the linear region of the transfer function of the first and second differential transistors. 
     
     
         6 . The transmitter driver according to  claim 5 , further comprising first and second capacitors in parallel with the first and second shifting resistors capable of increasing current from the third and fourth buffer transistors at increased frequencies. 
     
     
         7 . The transmitter driver according to  claim 1 , further comprising second and third degeneration resistors connected to third terminals of the first and second auxiliary transistors, respectively. 
     
     
         8 . The transmitter drive according to  claim 1 , further comprising a bias voltage connected to the second terminals of the first and second buffer transistors. 
     
     
         9 . The transmitter drive according to  claim 1 , further comprising a respective current source connected to each of the first and second buffer transistors, and the first and second differential pair transistors. 
     
     
         10 . The transmitter device according to  claim 9 , wherein the current sources connected to the first and second buffer transistors comprise variable current sources for adjusting the voltage at which the first and second auxiliary transistors turn on. 
     
     
         11 . The transmitter device according to  claim 10 , further comprising a feedback loop capable of sensing a bias voltage of the first and second buffer transistors, comparing the bias voltage to a reference voltage, and adjusting the current sources connected to the first and second buffer transistors until the first and second buffer transistors turn on at the maximum input voltage of the linear region of the transfer function of the first and second differential pair transistors. 
     
     
         12 . The transmitter device according to  claim 1 , further comprising first and second cascode transistors connected to the first and second differential pair transistors, respectively. 
     
     
         13 . The transmitter driver according to  claim 1 , wherein the first terminals comprise a base, the second terminals comprise a collector, and the third terminals comprise an emitter of a bipolar transistor. 
     
     
         14 . The transmitter driver according to  claim 1 , wherein the first terminals comprise a gate, the second terminals comprise a drain, and the third terminals comprise a source of a MOSFET. 
     
     
         15 . A transmitter comprising:
 the transmitter driver of  claim 1 ;   a laser for generating an optical signal; and   a modulator for modulating the optical signal in accordance with the first and second output electrical signals.   
     
     
         16 . The transmitter according to  claim 15 , wherein the buffer stage of the transmitter driver comprises first and second buffer transistors. 
     
     
         17 . The transmitter according to  claim 16 , wherein the buffer stage includes third and fourth buffer transistors including first terminals connected to the first terminals of the first and second buffer transistors, respectively, and third terminals connected to the first terminals of the first and second auxiliary transistors, respectively. 
     
     
         18 . The transmitter according to  claim 17 , further comprising first and second shifting resistors connected to the third terminals of the third and fourth buffer transistors, respectively, for shifting voltage applied to the first terminals of the first and second auxiliary transistors, respectively, whereby the first and second auxiliary transistors turn on when the first and second differential pair transistors exceed the maximum input voltage of the linear region of the transfer function of the first and second differential transistors.