A bias circuit is a portion of the device's circuit that supplies this steady current or voltage. The operating point of a device, also known as bias point, quiescent point, or Q-point, is the DC voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied. The AC signal applied to them is superposed on this DC bias current or voltage. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing time-varying ( AC) signals, also require a steady (DC) current or voltage at their terminals to operate correctly. In electronics, biasing is the setting of DC ( direct current) operating conditions (current and voltage) of an electronic component that processes time-varying signals. A graphical representation of the current and voltage properties of a transistor the bias is selected so that the operating point permits maximum signal amplitude without distortion. For the safety discharge resistor, see Bleeder resistor. Good luck, and post back with your simulation results."Bleeder bias" redirects here. Equivalents are BC349C, and BC849C (but its been a while so please check equivalency). This is an European transistor (made by Philips in its heyday), but OnSemi makes it as well. The "C" denotes the gain (A/B/C are ascending gain values for this transistor). Oh, and BTW, please do use the BC149C or BC549C for noise performance. Personally I do not like the extended low frequency response so I would add a series-C to R4 such that the cutoff lo-frequency is manageable. Try a gain of 50 first, so R4 is ~50 times R3. The ratio of R4/R3 sets the AC gain - just like a non-inverting op-amp, gain=(r4+r3)/r3. For R6/R7, I would start by making them equal (1K each?) and see where the DC bias lands with the collector voltage of TR2. Now, R5 is tricky - start with 3.3K and go up to 6.8K or so. R1 should be 100K (but in my experience, R2=R1 worked best, and I have used 100k to 220K depending on supply voltage and transistor gain). In my day we didn't do simulations - we just built the thing and tried different values! Here are some starting values (from memory) for simulations. Referring to that figure (3.2.6 in the link that I sent earlier) (using a 9V supply): Had you considered something like a parabolic reflector to improve the signal level (and pick out the birdsong from the other noises)? It's not hard to make a simple audio amplifier that's much lower noise than that and won't colour the signal. From memory, the noise bandwidth of an A-weighting filter is something like 10kHz, which means the noise voltage density at the output of the capsule will be in order of several tens of nV / sqrt(Hz). That's quite good for a cheap microphone, but the very best ones will be almost 10dB better (but you possibly wouldn't want to use one of those outside a studio). If I'm interpreting the spec sheet correctly (SNR 1kHz, 94dB (presumably SPL re 20uPa), A-weighted) is 80dB typical), the self noise of the capsule comes out to be about 14dB SPL. The output impedance and noise level are such that any following gain stage won't be that critical, and won't be what limits the performance. That capsule already contains a preamp (ok, it's just a FET).
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