- Design of Analog ICs in the context of electronic system design
- Deriving IC specifications from the host electronic system.
- A good understanding of CMOS devices & technology.
- Proficiency in analog circuit design and analysis.
- Mastery of design tools: ngspice, xscheme, magic, netgen & Python.
1. Introduction to an electronic system design, a plug-n-play USB-MIDI microphone.
- Microphone pre-amplifier and interface circuit design.
- Select an widely available Op-Amp for the preamplifier e.g. TI OPA 344
- Derive the important specs for the CMOS Op-Amp design.
2. Introduction to linear circuits and passive devices
- Understanding passive devices (RLC) using basic EM principles.
- Principle of linearity and superposition
- Network analysis: KCL, KVL, node theorems, Thevenin, Norton
- Emphasis on interfacing circuits and power transfer principle.
3. Basics of MOS device physics
- Introduction to pn junctions.
- MOS as capacitor.
- Threshold voltage.
- IV characteristics.
- Parasitic capacitance.
4. Basics of analog building blocks
- Current mirror design: simple, cascode and wide-swing mirrors
- Basic understanding of differential amplifiers.
- Introduction to AC analysis: stability analysis of a 2-stage amplifier.
- Design of a folded cascode amplifier using CMOS 130nm.
5. Implementation of the design
- All the designs will be done using Skywater 130nm CMOS technology.
- Schematic capture using open-source xschem.
- Simulation will be done using ngspice.
- Layout and final verification will be done using magic and netgen.
Calculating Thevenin Equivalent of Microphone
Key specs from the microphone Datasheet and research:
- Sensitivty: -44 dBV/Pa
- Condition: 94 dB SPL at 1 kHz which is sound pressure of 1 Pa
- Normal voice conversation is typically 60 dB SPL
-
Vth Calculation
- Voice (Pa) =
$10^{(60-94)/20} = 19.9\times 10^{-3} Pa$ - Output (Vpk) =
$\sqrt{2}\times V_{rms} = \sqrt{2}\times 19.9\times 10^{-3} Pa \times 10^{-44/20} = 178 \mu Vpk$ $V_{out-pk} = 0.178~ mV$
- Voice (Pa) =
- Rth (from datasheet) = 380 ohms.
Thevenin equivalent circuit :
Transfer Function:
From Sparkfun schematic:
- Rin=5k, Rfb=300k, therefore Gain = 60
- So output of the amplfier will be 60x0.178 mVpk = 10.68 mVpk
- Sparkfun site states 100 mVpk probaby assuming 10 times higher input signal i.e. Voice is 80 dB SPL
- Input high-pass frequency =
$1/2\pi RC = 1/2\pi 5k 4.7uF = 6.77 Hz$ - Feedback Low-pass filter frequency =
$1/2\pi RC = 1/2\pi 300k 27pF = 19.6kHz $ - Input common-mode filter =
$1/2\pi 10k 1uF = 15.9 Hz$
- AC simulation
- for a gain of 10000
- plot output voltage (in dB) and phase (in deg)
- measure the maximum gain and the frequency at the gain.
- measure the -3 dB frequency and verify with your calaculation.
MAX-------------------- 35.37dB
3db-------------------- 32.37dB
f_pole----------------- 19.73kHz
f_zero----------------- 6.63Hz
f_mid------------------ 19.5Hz
ph_pole---------------- -135`
ph_zero---------------- -224.9`
ph_mid----------------- -161.3`
MAX-------------------- 34.91dB
3db-------------------- 31.91dB
f_pole----------------- 20.76kHz
f_zero----------------- 6.3Hz
f_60------------------ 151.42MHz
for the transistor sizing
| Parameter | Increased by | Trade-offs |
|---|---|---|
| Gain | ↑ W, ↑ L | Area, power |
| Bandwidth | ↓ L, ↑ bias current | Noise, power |
| Output swing | ↑ L | Reduced speed |
| Slew Rate | ↑ W | More area, power |
| Noise | ↑ W | Larger area |
creating symbol for the OP-AMP
simulating with mic_test circuit
MAX-------------------- 21.83dB
3db-------------------- 18.83dB
f_pole----------------- 29.60kHz
f_zero----------------- 1.48Hz
Transient simulation
THD(Total Harmonic Distortion) = 28.52%
REVIEW :
- From the OPA-344 datasheet the gain is 120dB and the THD is 0.06%.
- For the common source differential amplifier which I have designed the gain is comming to be around 21dB which is very low.
- The THD is 28.52%