A Non-Switching Power Amplifier (Baldur)
Introduction
This amplifier is inspired by the article A new Class-AB design in Electronics-World-1999-12.
The output stage follows the design guide lines in the article, but the
input stage is totally different. I have called the amplifier Baldur, he was a
Norse god.
Principal schematic
The principal schematic is shown in the figure below. The amplifier is supplied
with the voltages V+ and V-. The input stage is a JFET with a source
resistor R6 and a constant current source I3 in the drain. The signal current I
In is fed to the bias control loop consisting of the transistors Q8,
Q9, Q10 and Q15. This provides that the bias of the power transistors Q17 and Q18 is
controlled by the constant current source I11. The drivers for the
power transistors are Q15 and Q16. Please observe that the power
transistors with their drivers Q15 and Q16 operate in common emitter,
and not in common collector as usual for most power amplifiers. This
has the drawback that the open loop output impedance is quite high,
making it necessary to have a fair amount of open loop gain avaliable
to provide a reasonable closed loop output impedance. The feedback is
accomplised by R19. I will not treat the bias control loop in detail. For a
more detailed analysis, please see the magazine article. A more
detailed analysis of mine can be found here.
The resistors R1 + R2 determine the input impedance. R1 has a
relatively low value, while the value of R2 can be quite high. The
bandwidth limitation on the input is mainly given by the JFET
capacitances in combination with R1 (and the output impedance of the
preceding stage).
The input JFET drain current is given by R6 and the transistor IDSS.
The current source of I3 has about the same drain current, since
normally the current drawn from the following stages are quite low.
This current source is trimable and is controlling the output offset
voltage.
The voltage to current gain in the input stage is approximately given by:
A1 = IiN/ViN =
gm/(1+gmR6)
Here gm is the transconductance of the JFET at the quiescent
current. Typically this can be about 5 mA with a transconductance
of about 30 mA/V. The current gain in the output Darlington (Q15/Q17
and Q16/Q18) is very high, about equal to the product between the
current gain of the drivers (β1) and the power transistors (β2). The gain from
the Darlington input to the output (OUT) with a load RL is then:
A2 =VOUT/IiN =
β1β2RL
Thus the open loop gain can be found to be:
A0 = gmβ1β2RL/(1+gmR6)
The closed loop gain is approximately:
AV = 1 + R19/R6
The bias is set by the constant current source I11 with IRef. From the theory in my analysis, this implies that:
IC15 = IC16 = √β
and:
IC17= IC18 = β2 IC15
Final schematic
The final schematic for the realized amplifier,
mounted on a PCB, is shown in the figure below. It is seen that the
constant current sources are realized by JFETs. The offset adjustment
is made by RV4 and the bias is set by RV12. It is also added the
components R13 and C14. R13 provides an open loop bandwidth of about 20
kHz while C14 make the amplifier stable down to a closed loop gain of
10 times (20 dB).
The amplifier is supplied
with the voltages VPOS and VNEG. These voltages can be chosen from the wanted output power.
As an example, for a 30 W into 8 ohms, the RMS voltage is:
V=√ PˇR =√
30ˇ8 =15.5 V
This corresponds to a peak value of:
Vp =15.5√ 2 =21.9 V
With a supply voltage of 25V, it is possible to get 30W out of this
amplifier before clipping. For a load of RL = 8 ohm, this corresponds to a peak value
for the current of:
Ip = Vp/RL = 21.9/8 = 2.74 A
When operating in Class A, since the output operates in Push-Pull, the quiescent current of
Q17/Q18 must be at least:
IC = Ip/2 =2.74/2 = 1.37 A
However, since this is a non-switching amplifier, it is tempting to
operate this amplifier in Class AB. The bias is set by the constant
current source with IRef.
It is also added an extra transistor Q7. This implies that:
IC15 = IC16 = IRef, IC18= IC19 = β2IRef
Please look at my analysis if you want an explanation.
A theoretical open loop gain A0 can be calculated to be
about 1000 times (60 dB) assuming a JFET transconductance of 30 mA/V, a
load of 8 ohms and current gain of 100 for the drivers and the power
transistors. This is in compliance with the simulated value. The
This amplifier can be supplied with power supply from about +- 10 V up
to +- 40 V. But the distortion of course increases when the output
voltage to the load increases, i.e. the output power increases. The
distortion is of couse lower if the amplifier has a high bias.
Printed Circuit Board (PCB)
The layout of the amplifier board is
shown in the figure below. This has the dimensions 100x52 mm. The
components in the diagram above are all located on the same circuit
board. J1-J5 are the connectors on the board. The transistors Q8, Q9,
Q10, Q15 and Q16 share the same heat sink made out of an aluminium
piece with the dimensions WxHxD like 40x20x10 mm. The
power transistors Q17 and Q18 must be mounted on a large heat sink, the
thermal resistance is dependent of the power dissipated and the bias
setting.
Resistor R0 isolates signal ground (GND) from power ground
(EARTH). This resistance value will typically lie in the range 4-10
ohms.
With 3D in KiCad, the printed circuit board looks like this:
Normally the power transistors Q17 and Q18 will be mounted horizontally as shown.
Power supply
As an example, the power supply can consist of, among other things, a
2x18 V 300 VA transformer (T1), common to both channels, see the figure
below. Separate rectifiers (D1 and D2) can be used for positive and
negative voltage. 47000 μF capacitors and 0.47 ohm power resistors for
the filtering can be considered reasonable. A fuse (F1) on the primary
side is a requirement. A circuit breaker is usually in series with this
fuse.
Build-up
The outputs from the transformer are fed to the rectifiers and on
to the CRC filtering (shown above). The ground of the phono socket is connected to
the screen of the phono cable, which is
connected to the amplifier board, to the point marked GND. The hot end
of the phono cable is connected to the amplifier board marked IN. From
the speaker output, the hot wire is led to the amplifier
board to the point marked OUT. The cold wire is connected to ground in
the power supply. From the power supply, the three connections are made
for the
power supply to the amplifier boards. All connections should be as
short as possible. If any kind of instability, noise or hum should
occur, it is highly likely that the cause is poor wiring (leading to,
for example, earth loops).
It is recommended to use a variable mains transformer when starting the
amplifier for the first time. As the voltage supply increases, the
offset is adjusted using potentiometer RV4. The bias is adjusted with the potentiometer RV12. If possible use an
oscilloscope to look at the output, there should be nothing but noise
here if everything is fine. As the temperature increases, it may be
necessary to readjust the offset (and the bias if necessary).
Bill of Materials (BOM) for one channel is shown below. The amplifier is well suited
for personal adaptations. For replacements, remember to take into
account changed physical dimensions and pin configurations, especially
for the use of other transistor types when mounting on the circuit
boards.
Bill of Material for one PCB
Metal film resistors with 1% tolerance have been used. Other types are
of course also possible as long as they fit in the card.
R0 10 ohm
R1 1 kohm
R2 100 kohm
R6 47 ohm
R13 330 kohm
R19 470 ohm
RV4 100 ohm Potentiometer Bourns 3296W
RV12 200 ohm Potentiometer Bourns 3296W
C14 100 pF NP0/C0G P 5.0mm
C20, C21 10u Radial Film L18.0mm W9.0mm P15.0mm
Q7, Q9, Q10, Q15 KSA1381 TO-126
Q8, Q16 KSC3503 TO-126
Q17 KSA1943 TO-247
Q18 KSC5200 TO-247
J1 Screw terminal 01x02
J2-J5 Solder terminals
HS1 Heatsink
Please notice:
This project
description is for non-commercial use, only. Using this document on a
site and charging a fee for download is vialation of non-commercial use
and prone to demand for payment. So, for commercial use, contact me for
agreement of terms. This page, however, can
be downloaded for own use, and linked to, not violating term of
non-commercial use.
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Knut Harald Nygaard
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