Circuit and method for biasing an amplifier
An amplifier circuit includes an amplifier core having a cascode transistor and a gain transistor, a bias circuit coupled to the amplifier core, the bias circuit comprising: a first current source, a second current source, an operational transconductance amplifier (OTA), a bias cascode transistor pair having a bias cascode transistor and a bias gain transistor, and a replica circuit coupled to the first current source and to the second current source.
1. An amplifier circuit, comprising:
an amplifier core having a cascode transistor and a gain transistor;
a bias circuit coupled to the amplifier core, the bias circuit comprising:
a first current source;
a second current source;
an operational transconductance amplifier (OTA);
a bias cascode transistor pair having a bias cascode transistor and a bias gain transistor; and
a replica circuit coupled to the first current source and to the second current source, the replica circuit comprising:
a first replica transistor coupled to the first current source, and a second replica transistor coupled to the second current source;
a third replica transistor coupled to the second replica transistor;
an adjustable resistance coupled to the first replica transistor;
a voltage divider coupled to the second replica transistor and the third replica transistor, wherein an output of the voltage divider is coupled to a gate of the third replica transistor to provide a bias voltage, Vgate, for the amplifier core.
2. The amplifier circuit of claim 1 , wherein the voltage divider comprises a first resistance and a second resistance, the gate of the third replica transistor coupled to a first node between the first resistance and the second resistance.
3. The amplifier circuit of claim 2 , wherein the first resistance is coupled between the first node and a system ground.
4. The amplifier circuit of claim 2 , wherein the second resistance is coupled between the first node and a drain of the second replica transistor.
5. The amplifier circuit of claim 1 , wherein the first replica transistor has a size (W1/L1) and the second replica transistor has a size N1*(W1/L1).
6. The amplifier circuit of claim 5 , wherein the third replica transistor has a size (W/L).
7. The amplifier circuit of claim 1 , wherein a second node is formed between a source of the second replica transistor and a drain of the third replica transistor.
8. The amplifier circuit of claim 7 , wherein the OTA has a non-inverting input coupled to the second node, an inverting input coupled to a third node formed between a source of the bias cascode transistor and a drain of the bias gain transistor.
9. The amplifier circuit of claim 8 , wherein an output of the OTA is a cascode bias signal applied to a gate of the bias cascode transistor and a gate of the cascode transistor.
10. The amplifier circuit of claim 1 , wherein the amplifier core is a low noise amplifier (LNA) core.
11. The amplifier circuit of claim 1 , wherein:
the replica circuit comprises a first current loop; and
the OTA comprises a second current loop.
12. The amplifier circuit of claim 11 , wherein the replica circuit is configured to generate a first bias current without an operational amplifier.
13. The amplifier circuit of claim 11 , wherein the first current loop generates a first bias current for the amplifier core that is proportional to a gain of the gain transistor.
14. The amplifier circuit of claim 1 , wherein the voltage divider comprises a resistive feedback structure.