Test circuit and method

1. A test circuit comprising:

an amplifier configured to receive an alternating current (AC) signal, and output an amplified AC signal based on the AC signal;

a first detection circuit configured to generate a first direct current (DC) voltage having a first value based on an amplitude of the AC signal; and

a second detection circuit configured to generate a second DC voltage having a second value based on an amplitude of the amplified AC signal.

2. The test circuit of claim 1 , further comprising an oscillator coupled in series with an isolation circuit, wherein the isolation circuit is configured to output the AC signal based on an oscillation signal of the oscillator.

3. The test circuit of claim 2 , wherein

the oscillator comprises a ring oscillator, and

the isolation circuit comprises a low-pass filter.

4. The test circuit of claim 2 , wherein

the oscillator comprises an inductor-capacitor (LC) resonator, and

the isolation circuit comprises a buffer.

5. The test circuit of claim 1 , wherein the amplifier comprises a variable-gain amplifier comprising a switched PMOS array coupled to a switched NMOS array.

6. The test circuit of claim 1 , wherein the amplifier comprises a common-source configuration of a transistor and an inductive load.

7. The test circuit of claim 1 , wherein each of the first and second detection circuits comprises a cascade arrangement of gain stages in series with a low-pass filter.

8. The test circuit of claim 1 , wherein

the first detection circuit is capacitively coupled to an input terminal of the amplifier, and

the second detection circuit is capacitively coupled to an output terminal of the amplifier.

9. An integrated circuit (IC) comprising:

an amplifier coupled between a first node and a second node;

a first detection circuit coupled between the first node and a first pad positioned in a scribe line of a semiconductor wafer; and

a second detection circuit coupled between the second node and a second pad positioned in the scribe line,

wherein

the amplifier is configured to generate an amplified alternating current (AC) signal at the second node based on a variable gain and an AC signal on the first node,

the first detection circuit is configured to output a first direct current (DC) voltage to the first pad, the first DC voltage having a first value based on an amplitude of the AC signal, and

the second detection circuit is configured to output a second DC voltage to the second pad, the second DC voltage having a second value based on an amplitude of the amplified AC signal.

10. The IC of claim 9 , further comprising an oscillator coupled to a third pad positioned in the scribe line, wherein the oscillator is configured to generate the AC signal on the first node responsive to a third DC voltage on the third pad.

11. The IC of claim 9 , wherein

the amplifier is coupled to a third pad positioned in the scribe line, and

the amplifier is configured to control the variable gain based on a third DC voltage on the third pad.

12. The IC of claim 9 , wherein the amplifier comprises an inductive load comprising a meander or transmission line positioned in the scribe line.

13. The IC of claim 9 , wherein each of the first and second detection circuits comprises:

a first common-source stage coupled to an input terminal through a capacitive device;

a second common-source stage coupled to the first common-source stage; and

a low-pass filter coupled between the second common-source stage and an output terminal.

14. A method of measuring an alternating current (AC) amplifier gain, the method comprising:

using an AC amplifier to generate an amplified AC signal from a received AC signal;

outputting a first DC voltage to a first pad of a semiconductor wafer, the first DC voltage having a first value based on an amplitude of the AC signal; and

outputting a second DC voltage to a second pad of the semiconductor wafer, the second DC voltage having a second value based on an amplitude of the amplified AC signal.

15. The method of claim 14 , further comprising calculating a gain value from the first and second values.

16. The method of claim 14 , further comprising setting a gain of the AC amplifier by providing a third DC voltage to a third pad of the semiconductor wafer.

17. The method of claim 14 , further comprising generating the AC signal using a ring oscillator or an inductor-capacitor (LC) resonator.

18. The method of claim 14 , further comprising electrically accessing the first and second pads as part of performing a wafer acceptance test (WAT) operation on the semiconductor wafer.

19. The method of claim 14 , wherein

the AC signal comprises a radio frequency (RF) signal, and

the using the AC amplifier to generate the amplified AC signal comprises using a push-pull circuit structure.

20. The method of claim 14 , wherein

the first and second pads are positioned in a scribe line of the semiconductor wafer,

the AC signal comprises a millimeter-wave signal, and

the using the AC amplifier to generate the amplified AC signal comprises using a common-source stage with an inductive load, the inductive load comprising a conductive line positioned in the scribe line.