# Bjt Amplifier Design - Clarification on Specs

Hi,

Just having trouble unpacking the question attached. I know that Vcc = 12, Zout of Voltage/Pre amp = 600 ohms, f = 20Hz to 20kHz, Zin = 47k ohms. However, I'm having a but of trouble understanding what my output is. Is it 2mV or 2V Rms (2.83V peak)? And could you clarify any information above that I may have misinterpreted?

Thanks.

• I see you just deleted your earlier question. And no, the Zout of the preamp is NOT 600 Ohms. That's the input impedance of the power amplifier you'll be driving. Different things. Also, your voltage gain will be about 63 dB. I take it you no longer need the earlier question answered and now have yet another one that you don't know how to approach.
– jonk
Commented Jun 4, 2020 at 22:39
• If this was for real, the 2mV peak output would indicate that the signal came straight from the pickup. In which case you would need to build an RIAA equaliser into the preamp. Fortunately, this is just homework so it doesn't have to work.
– user16324
Commented Jun 4, 2020 at 23:01
• @BrianDrummond Crap. I forgot about that detail. And yes, you are right about that. In fact, I seem to recall that there was plenty of confusion at the time as there were several popular record coding schemes and not only one. So a "decent" pre-amp might include several settings.
– jonk
Commented Jun 4, 2020 at 23:17
• Where do you connect the output of the record player, and where do you connect the input of the power amp? Commented Jun 5, 2020 at 0:31
• Where do you connect the output of the record player, and where do you connect the input of the power amp? Commented Jun 5, 2020 at 0:31

A bipolar gain device, with resistive load with 12 volt VDD, can only provide 12/0.026 == 480X gain.

Thus you need two gain stages, plus buffer stage into power amp.

For good signal-noise-ratio (if 80 dB is adequate), you need 0.2 microvolts total random noise RTI referred to input.

In 10,000Hz bandwidth, need about 2 nanoVolt ( 0.002 uv) noise density; this requires a total input_referred Rnoise of 90 ohms. A bit of a challenge.

Interestingly, RIAA preamplifiers are easier to design, because their bandwidth is a mere 50Hz, with that 50Hz rolloff continuing cleanly out past 10,000 Hz. With 20X slower bandwidth, the Rnoise can be 20X higher, what with

• Vnoise = sqrt( 4 * K * T * Bandwidth * Resistance)

Note the tradeoff between Bandwidth and Resistance.

ahhhh and the power supply rejection of standard resistor-load-gain-stages is approximately ZERO. Thus a very clean LDO is needed.

==================================

considering RIAA:

a) use GAIN=30DB, RIAA_ROLLOFF, GAIN=30/50DB, OUTPUT_BUFFER to have the vinyl energy at 20,000 Hz be strongly attenuated; the diyAudio preamp uses 30 volt supplies and NJFET devices

b) planning the PCB to avoid echoes, caused by delays through the 50Hz primary pole with output_to_input feedback in the PCB, requires a LONG aspect_ratio PCB; to adequately reduce out_in crosstalk; each square of foil in the ground will give about 12dB attenuation; when the circuit has 60 to 80 dB gain, the physical layout must give attention to out_in crosstalk within the ground system; without echo_suppression to -120 dB (or at least to -100 dB), the music gets muddy what with inter_tonal time crosstalk.

c) diyAudio "simplistic NJFET RIAA" thread gives much attention to shunt regulator with just a few NANOVOLTS of noise; granted the Vdd design uses numerous large capacitors to handle high frequency trash.

• The RIAA curve isn't quite that simple, there is a flattening mid-band then a further rolloff (which does continue past 10 kHz). But that doesn't affect the basic premise of your answer. However, note the "2mV peak" is generally measured at 1 kHz, in practice you need an additional 20dB gain at 20 kHz.
– user16324
Commented Jun 5, 2020 at 12:06
• Combine the 40dB gain variation across the band with decent headroom for large amplitude HF signals and an RIAA preamp is actually quite challenging.
– user16324
Commented Jun 5, 2020 at 12:19