Amplifying circuit

A differential amplifier comprising:

• (1) first and second amplifying elements having grounding electrodes are connected to each other so as to operate differentially in response to an input signal;
• (2) first and second additional amplifying elements having grounding electrodes connected to the output electrodes of said first and second amplifying elements and whose input electrodes are connected to a predetermined DC potential;
• (3) first and second load means whose ends on one side are connected to the output electrodes of said first and second additional amplifying elements and whose ends on the other side are connected to a first operating potential;
• (4) a common impedance means connected between a second operating potential and said grounding electrodes of said first and second amplifying elements;
• (5) a first current by-passing means connected to a common connection node of said output electrode of said first amplifying element and said grounding electrode of said first additional amplifying element;
• and
• (6) a second current by-passing means connected to a common connection node of said output electrode of said second amplifying element and said grounding electrode of said second additional amplifying element.

An optical communications system comprising:

• (1) an optical transmission path which receives an optical input signal through an end thereof and which sends an optical output signal through the other end thereof;
• (2) a photoelectronic converting means which converts said optical output signal into an electric signal;
• and
• (3) a multi-stage amplifier for amplifying said electric signal;
• wherein the amplifier of at least one stage in said multi-stage amplifier is constituted by a differential amplifier comprising:
• first and second amplifying elements whose grounding electrodes are connected to each other so as to operate differentially in response to an input signal,
• first and second additional amplifying elements whose grounding electrodes are connected to the output electrodes of said first and second amplifying elements and whose input electrodes are connected to a predetermined DC potential,
• first and second load means whose ends on one side are connected to the output electrodes of said first and second additional amplifying elements and whose ends on the other side are connected to a first operating potential,
• a common impedance means connected between a second operating potential and said grounding electrodes of said first and said second amplifying elements,
• a first current by-passing means connected to a common connection node of said output electrode of said first amplifying element and said grounding of said first additional amplifying element, and
• a second current by-passing means connected to a common connection node of said output electrode of said second amplifying element and said grounding electrode of said second additional amplifying element,
• wherein said first and second current by-passing means are constituted by first and second constant-current source circuits, respectively, and said common impedance means is constituted by a third constant-current source circuit,
• wherein a signal of the common connection node of said one end of said first load means and said output electrode of said first additional amplifying element is input to a first impedance converting means that has a high input impedance and a low output impedance, and a signal of the common connection node of said one end of said second load means and said output electrode of said second additional amplifying element is input to a second impedance converting means that has a high impedance and a low output impedance, and
• wherein said first and second amplifying elements, as well as said first and second additional amplifying elements are MESFETs in a GaAsIC, and wherein said first impedance means includes a first source follower MESFET and the second impedance means includes a second source follower MESFET.