EDNE September 2012 - (Page 59)

designideas readerS SOLVe deSIGN PrOBLeMS Condenser microphone uses dc-coupled impedance converter Dimitri Danyuk, Miami, FL DIs Inside 60 Simple sawtooth generator ↘ The diaphragm of a condenser microphone is the movable plate of a capacitor. With a polarized capacitor, the vibration of the diaphragm in relation to the back plate produces an ac audio-output voltage. The condenser capsule has a capacitance of 10 to 60 pF; thus, you should connect it to an impedance converter with extremely high input impedance for a flat frequency response. The conventional impedance converter is a JFET source follower with an additional amplifying and power- decoupling circuit. You supply power to the impedance-converter circuitry using the same microphone-cable conductors that carry the audio signal. The balanced audio pair at the XLR connector’s pins 2 and 3 both carry the same positive dc voltage, or phantom power, relative to Pin 1’s ground. The amplifying/decoupling circuit contains an audio transformer or a couple of capacitors to separate the dc power from the audio signal. High-value dc-blocking capacitors can generate measurable and audible operates at high frequency 62 CMOS gate makes long-duration timers using RC components 63 Inverting level-shift circuit has negative potential ▶To see all of EDN’s Design Ideas, visit www.edn.com/designideas distortion (Reference 1). Microphone circuits rarely use the highest-quality capacitors because of space limitations. 48V PHANTOM POWER R6 75 1 J1 2 3 + GND − XLR SHIELDED TWISTEDPAIR CABLE 2 1 3 1 2 3 + 1 J2 2 3 + GND − XLR RPH1 6.81k RIN RPH2 6.81k TO PREAMP 4 mA R7 75 4 mA C1 1000 pF G X1 D Q1 S Q3 C3 1 F BCV46 R3 1M R5 5.1k BCV46 Q4 0.5 mA R4 5.1k R1 2k C2 1 F Q2 BCV46 R2 1M 6 4 7 − 2 D1 BAT54S C4 10 F + IC1 TL061 D2 20V +3 Figure 1 A feedback loop balances the dc voltage on the paired cable conductors, which power high- to low-impedance converter Q1. www.edn-europe.com SEPTEMBER 2012 | EDN EuropE 59 http://www.edn.com/designideas http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE September 2012

Cover
Contents
International Rectifier
Microchip
RS Components
Masthead
Microchip
EDN Comment
Pulse
Analog Devices
Altera
Baker's best
Messe München
Test & Measurement World
Agilent Techno
Digi-key
Bergquist
Advanced power switches boost microhybrid emissions gains
Digi-Key
Image sensors evolve to address Emerging embedded- vision needs
Renesas
Silicon Labs
Digi-Key
Vicor
Power becomes a software issue as smart phones become smarter
IAN
Power : a significant challenge in EDA design
Digi-Key
Mechatronics in Desin
Design Ideas
Product roundup
Tales from the Cube

EDNE September 2012