Monolithic integrated differential amplifier with digital gain setting

A differential amplifier having means for adjusting the gain over a wide bandwidth, comprising:

• an operational amplifier having an input terminal and an output terminal, wherein said operational amplifier includes:
• a first transconductance amplifier as an input stage, said first transconductance amplifier further including a differential stage comprising a first p-channel transistor and a second p-channel transistor coupled to said first p-channel transistor by a common source terminal, wherein said common source terminal is supplied with a first source current, said first p-channel transistor and said second p-channel transistor have fixed channel lengths and fixed channel widths, wherein the transconductance of said first transconductance amplifier is variable in steps by means of at least one paralleling stage which is identical in design to said differential stage of said first transconductance amplifier;
• a second transconductance amplifier as an output stage, said second transconductance amplifier including an input terminal and an output terminal, said second transconductance amplifier being coupled to said first transconductance amplifier;
• and
• a negative feedback network coupled between said input terminal and said output terminal of said second transconductance amplifier, wherein said negative feedback network determines the frequency response of said operational amplifier;
• a switching means coupled to said input terminal of said operational amplifier for varying the resistance appearing at said input terminal of said operational amplifier;
• and
• an adjusting means coupled to said switching means for varying the gainbandwidth product of said operational amplifier according to the input resistance selected by said switching means.

A differential amplifier, comprising:

• first and second resistor chains, each chain having a plurality of resistive elements separated by taps, said first chain having a first chain input terminal and a first chain output terminal and said second chain having a second chain input terminal and a second chain output terminal;
• a first switching means having a plurality of inputs, each input coupled to an associated one of said taps of said first resistor chain, wherein any desired one of said inputs can be selected to couple to a first common output terminal of said first switching means;
• a second switching means having a plurality of inputs, each input coupled to an associated one of said taps of said second resistor chain, wherein any desired one of said inputs can be selected to couple to a second common output terminal of said second switching means;
• an operational amplifier having an inverting input terminal, a noninverting input terminal, and an amplifier output terminal, wherein said inverting input terminal is coupled to said first common terminal of said first switching means, said noninverting input terminal is coupled to said second common terminal of said second switching means, and said amplifier output terminal is coupled to said first chain output terminal of said first resistor chain;
• control means further coupled to said first and second switching means and said operational amplifier, wherein said control means switches said first and second switching means to couple at least one of said taps in each of said first and second resistor chains to said inverting and noninverting input terminals of said operational amplifier to achieve a desired gain, and switches the gain-bandwidth product of said operational amplifier according to said taps selected, thereby causing said operational amplifier to operate over a wide-bandwidth irrespective of its gain.