Thanks Russell. I am not sure I understood it all. What was the bit after " RE: electret microphone preamp" ? Just kidding. I will try some of your ideas. Steve -----Original Message----- From: Russell McMahon To: PICLIST@MITVMA.MIT.EDU Date: Thursday, February 04, 1999 8:01 AM Subject: Re: [OT] electret microphone preamp >1. Mike's simple circuit is OK but is rather subject to the beta >of the transistor. >In practice it may well meet your need. >See at end for complexish ravings on gain of original circuit. > >2. All transistors are not created equal as regards noise - if you >have a choice choose a type specified as "low noise" (could be the >start of a lovely argument here). > >3. The circuit's distortion performance can be improved >dramatically by placing a small resistor (approx 5 to 10% of >collector resistor) between the emitter and ground and bypassing it >with a 10uF electrolytic capacitor Ce. >Chances are, with the Isd's ~ 8 bit accuracy, this won't matter. > >4. Gain: >Believe it or not, the gain of a one transistor circuit (simple or >modified) is about > > (Vcc - Vcollector)*40 > >Where Vcollector is the DC voltage at the collector in the absence of >input signal. >As this is fairly fixed with the 2 resistor biasing system, regadless >of beta, the gain does not vary with transistor characteristics. >This expression applies to a circuit with emitter to ground OR with >the emitter resistor bypassed with a capacitor. >"Why" is probably covered in "The art of electronics" (which I have >not got so it may not be).. > >5. A more complex and (only possibly :-) ) better circuit follows: > >Rc == R3 etc > >More controlled gain can be had by using 2 resistors from the base >instead of R2 - R2 now goes to Vcc and a new R4 to ground with each >very roughly around 100k.(actual values to set Vbase and thus Ve and >indirectly Vc - see below), plus an emitter resistor Re. In this >circuit Re MUST be used . > > If this diagram looks a mess and you are a Windoze user, >copy into Notepad. > If it still looks a mess it may be :-) > > > Vcc Vcc Vcc > | | | > | | R3 > | | | > | | *-------C2---- Out > R1 | | > | R2 | C > | | |/ > *---C1---*--| > | | |\ > MIC R4 > E___ > | | RE | > GND GND | Ce > GND Gnd > >Re ~ Rc/10 say (eg Rc = 10K and Re = 1K - could be less) >Decide drop across Rc to set desired gain - say 2.5v for gain of 100 >(From above: Gain = Vcc * (5-2.5) = 100) >Drop across Re will be 10% of this (as both share same >current)(almost) >So Vre = 0.25v >Vb is 0.6v more than Ve so Vb = 0.25+0.6 = 0.85 volt. >Choose R2, R4 to set Vb = 0.85 volt. >VR4 = 0.85v >VR2 = Vcc-0.85 = 5-0.85 = 4.15V >R's are in approx proportion to voltage drops (ignoring here >transistor base current). >R2:R4 ~ 4.15:0.85 ~ 5:1 >Say R2 = 100K, R4 = 22K >Some playing will still be required but the gain will be variable to >suit over a wide range and will be moderately stable once set. (For a >10K Rc The base current will be about 1uA for a beta of 300ish and >will affect the result somewhat) > >Gain = (Vcc - Vcollector)*40 as above > >Where Vcollector is the DC voltage at the collector in the absence of >input signal. >As this is fairly fixed with the 2 resistor biasing system, regadless >of beta, the gain does not vary with transistor characteristics. > > > >6. GAIN OF ORIGINAL CIRCUIT > >Only for the pathologically enthused: > >B = beta >Rc = collector resistor >Rb = base resistor >Gain ~ 40 * (Vcc - Vc) >Vc = (VccRb + 0.6.B.Rc)/(Rc.B + Rb) > >The following table shows the approximate gain which you will get >with the simple circuit for various values of Rb, the base resistor, >and beta (transistor current gain). I have used the exact table >values in the text so they can be compared to the table. Actual gains >will be of this order. > >As may be seen, for low values of beta (say around 100) (such as an >el- cheapo junk box transistor MAY provide, the gain may be varied >quite widely (56 to 165) by varying Rb. >For high betas (300 is entirely typical) the gain only varies from >147 to 194 as Rb is changed from 600,000 ohms to 50,000 ohms. > >Table was made in Excel >Displaying in non-proportional font MAY help get columns aligned. > >Vcc = 5 >Rc = 4700 > > >Beta 50 100 150 200 250 300 350 400 450 500 >Rb Gains >50000 165 181 187 190 192 193 194 195 195 196 >100000 140 165 175 181 184 187 189 190 191 192 >150000 122 152 165 172 177 181 183 185 187 188 >200000 108 140 156 165 171 175 178 181 183 184 >250000 97 131 148 158 165 170 174 177 179 181 >300000 88 122 140 152 159 165 169 172 175 177 >350000 80 115 134 146 154 160 165 169 172 174 >400000 74 108 128 140 149 156 161 165 168 171 >450000 69 102 122 135 145 152 157 161 165 168 >500000 64 97 117 131 140 148 153 158 162 165 >550000 60 92 112 126 136 144 150 155 159 162 >600000 56 88 108 122 132 140 147 152 156 159 > >eg for Rb = 100K and beta = 200, gain = 181 (vvv approximately) > > >-----Original Message----- >From: Mike Keitz > >>On Wed, 3 Feb 1999 11:08:25 +1100 Steve Ridley > >>writes: >>>So I now need a simple but >>>low noise if possible, preamplifier for an electret microphone. >>>Something using an op-amp or just a transistor would be fine. >>> The input impedance of the ISD33000 analog input is 3k and it >takes a maximum of 32 mV p-p >>>when fed in single ended input mode. >> >>If you have an extra op-amp, may as well use it. Otherwise this >>deceptively simple transistor circuit should meet your needs: >> >> Vcc Vcc >> | | >> | R3 >> | | >> | -----*-------C2---- Out >> R1 | | >> | R2 | C >> | | |/ >> *---C1---*--| >> | |\ >> MIC > E >> | | >> GND GND >> >> >>The transistor is any old NPN you can find. To start with, try 4.7K >for >>R1 and R3 and 470K for R2. C1 and C2 should be at least 0.1 uF. >Higher >>capacitance will increase bass response but that may not be what you >>want. If you have a 3-wire microphone, R1 and maybe C1 are built >into >>the microphone itself. >> >>If you have too much gain, make R2 smaller. This also changes the >DC >>bias on the transistor, but since only 32 mV of output is needed it >is OK >>to operate the transistor close to saturation. You can also add a >>resistor in series with C1 to reduce the gain. If someone will be >>speaking directly into the microphone, it may be able to drive the >ISD >>chip without an amplifier at all. >> etc >