Radio frequency (RF) equalizer in an envelope tracking (ET) circuit
A radio frequency (RF) equalizer in an envelope tracking (ET) circuit is disclosed. A transmitter chain includes an ET circuit having an RF equalizer therein. The RF equalizer includes a two operational amplifier (op-amp) structure that provides a relatively flat gain and a relatively constant negative group delay across a frequency range of interest (e.g., up to 200 MHz). The simple two op-amp structure provides frequency response equalization and time tuning adjustment and/or creates a window Vcc signal.
1. An envelope tracking (ET) circuit comprising:
an equalizer comprising a first branch, the first branch comprising:
a positive differential input node;
a negative differential input node;
a first operational amplifier (op-amp) comprising:
a first negative input coupled to the positive differential input node; and
a first output;
a first feedback circuit coupling the first output to the first negative input;
a second op-amp comprising:
a second negative input coupled to the negative differential input node and the first output; and
a second output; and
a second feedback impedance network coupling the second output to the second negative input.
2. The ET circuit of claim 1 , wherein the first feedback circuit comprises a resistor.
3. The ET circuit of claim 1 , wherein the second feedback impedance network comprises:
a first resistor;
a second resistor coupled to the first resistor with a node therebetween; and
a capacitor coupling the node to ground.
4. The ET circuit of claim 1 , further comprising a capacitor connected to the first output and the second negative input.
5. The ET circuit of claim 1 , further comprising a first input circuit coupled to the first negative input and comprising a resistor and a capacitor electrically parallel to one another.
6. The ET circuit of claim 1 , further comprising a second input circuit coupled to the second negative input and comprising a resistor.
7. The ET circuit of claim 1 , wherein the first op-amp further comprises a positive input coupled to ground.
8. The ET circuit of claim 1 , wherein the second op-amp further comprises a positive input coupled to ground.
9. The ET circuit of claim 1 , wherein the second feedback impedance network comprises:
a first resistor;
a second resistor coupled to the first resistor with a node therebetween;
a capacitor coupled to the node; and
a third resistor coupling the capacitor to ground.
10. The ET circuit of claim 1 , further comprising a second branch, the second branch comprising:
a second positive differential input node;
a second negative differential input node;
a third op-amp comprising:
a third negative input coupled to the second negative differential input node; and
a third output;
a third feedback circuit coupling the third output to the third negative input;
a fourth op-amp comprising:
a fourth negative input coupled to the second positive differential input node and the third output; and
a fourth output; and
a fourth feedback impedance network coupling the fourth output to the fourth negative input.
11. A transmitter circuit comprising:
a transceiver;
an amplifier network coupled to the transceiver; and
an envelope tracking (ET) circuit coupled to the transceiver and the amplifier network,
the ET circuit comprising:
an equalizer comprising:
a positive differential input node;
a negative differential input node;
a first operational amplifier (op-amp) comprising:
a first negative input coupled to the positive differential input node; and
a first output;
a first feedback circuit coupling the first output to the first negative input;
a second op-amp comprising:
a second negative input coupled to the negative differential input node and the first output; and
a second output; and
a second feedback impedance network coupling the second output to the second negative input.
12. An envelope tracking (ET) circuit comprising:
an equalizer comprising:
a positive input node;
a negative input node;
a first operational amplifier (op-amp) comprising:
a first negative input coupled to the negative input node;
a first negative output;
a first positive input coupled to the positive input node; and
a first positive output;
a first feedback circuit coupling the first positive output to the first negative input;
a second feedback circuit coupling the first negative output to the first positive input;
a second op-amp comprising:
a second negative input coupled to the first positive output;
a second positive input coupled to the first negative output;
a second negative output; and
a second positive output;
a first feedback impedance network coupling the second negative output to the second positive input;
a second feedback impedance network coupling the second positive output to the second negative input node; and
a third impedance network coupling the second positive input to the negative input node.
13. The ET circuit of claim 12 , further comprising a fourth impedance network coupling the second negative input to the positive input node.
14. The ET circuit of claim 12 , further comprising a first input circuit serially positioned between the negative input node and the first negative input.
15. The ET circuit of claim 14 , further comprising a second input circuit serially positioned between the positive input node and the first positive input.
16. The ET circuit of claim 12 , wherein the first feedback impedance network comprises:
two serially coupled resistors with a node therebetween, the node coupled to ground through a variable capacitor; and
a second variable capacitor electrically parallel to the two serially coupled resistors.
17. The ET circuit of claim 12 , wherein the third impedance network comprises a variable capacitor and a resistor, the resistor electrically parallel to the variable capacitor.
18. The ET circuit of claim 17 , further comprising a fourth impedance network, wherein the fourth impedance network comprises a second variable capacitor and a second resistor, the second resistor electrically parallel to the second variable capacitor.
19. The ET circuit of claim 12 , further comprising a variable capacitor electrically positioned between the first negative output and the second positive input.
20. A method of operating an envelope tracking circuit, comprising:
receiving a differential signal at a positive differential input node and a negative differential input node;
sending signals from the positive differential input node to a first negative input of a first operational amplifier (op-amp);
providing a first feedback signal through a first feedback circuit from a first output of the first op-amp to the first negative input;
sending signals from the negative differential input node to a second negative input of a second op-amp;
coupling the first output to the second negative input;
providing a second feedback signal through a second feedback impedance network from a second output of the second op-amp to the second negative input.