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Fet Buffer for amplifiers


The EDFET drives like a FET, but with the bias stability of bipolar. Amps of output current can be controlled by milliamps of input current. The current gain is a design choice dictated by bandwidth. Two of things you have to consider when adding a power output stage to an op-amp circuit are the frequency response and the cross-over distortion in that stage.

This is especially true with wide band amplifiers, where the unity gain crossover needs to be at several hundred kilohertz. The stage is driven much the same as a complimentary pair output stage, but with the current gain that comes with using FETs., and with feedback within the output stage that that extends the buffer's bandwidth and regulates the quiescent current. More predictable operation allows the designer to design a circuit lower overall power dissipation and better closed loop stability.

Fet Buffer for amplifiers

The EDFET complimentary buffer is made up of a pair of unity gain buffers, one that drives in the positive direction and the other that drives in the negative direction. Pictured above is the positive driving half of the output stage.

Gain to make the output signal track the input signal comes from inverting transistor, Q1. The input signal is applied to the emitter of Q1 and the output of the amplifier is raised one diode drop to match the forward base-emitter drop of Q1, by diode connected transistor Q2. The buffer's offset is determined by the log of the magnitude of the mismatch in the emitter currents in Q1 and Q2, and it is directly proportional to the absolute temperature.

Since the saturation current usually isn't published for the transistors this expression is only usefully for appreciating the dependence of junction voltage on current and temperature. You can come up with your own value of I0 for a given transistor if you know all the other parameters and solve the above formula for I0. By the way, since, for most practical uses, you will be running at more than a thousand times the saturation current, the "+1" term can be dropped from practical calculations.

As an example, for the audio amplifier using a EDFET buffer shown in Figure 1. The following assumptions are applied: The maximum output voltage is 5 VDC with respect to ground, the power supply (VA) is 12 VDC, the maximum gate voltage is 8 VDC, the input capacitance, Ciss of the BUZ73 is 500 pf, and an...

Discrete Buffer: Diamond Buffer
Discrete Buffer: JISBOS Buffer

Design Buffers: Improved unity-gain follower delivers fast, stable response

Robert A Pease, National Semiconductor Corp -- EDN, June 27, 2011

Heavy load capacitance can cause the output of a unity-gain follower—an operational amplifier with direct feedback to the inverting input (Fig 1)—to ring and oscillate. The LM110 follower, for example, normally drives a 50-pF load without problems, but it does not drive 500 pF stably—high capacitance significantly modifies the open-loop output impedance, reducing the phase margin to zero and causing oscillation.

You can easily eliminate such instability problems by adding a capacitor and resistor in series across the op amp's inverting and noninverting inputs. This solution can also greatly improve a follower's slew rate.

Analyze the problem

In general, increasing the ac noise gain of an op amp's feedback network improves capacitive-load tolerance. A common gain-increasing strategy adds R2~RF/10 to the circuit shown in Fig 2. (A moderate-value capacitor, C2, usually inserted in series with R2, prevents the dc noise gain from also increasing and degrading dc-offset, drift and accuracy specifications.)

Improved unity-gain follower delivers fast, stable response figure 2

If the op amp has a 1-MHz gain-bandwidth product and R1=RF, the closed-loop frequency response will be 500 kHz. Inserting R2=RF/10 drops this frequency response to 90 kHz, where the amplifier usually tolerates a much larger capacitive load. AC noise gain equals (RF/ R1)+(RF/R2)+1, and dc noise gain is (RF/R1)+1.
Improved unity-gain follower delivers fast, stable response figure 2

You can also increase ac noise gain by installing R3 and C3 instead of R2 and C2. The resulting value is


In the simplest case, R1 forms an open circuit, and ac noise gain equals


Therefore, you can raise ac noise gain by using a low value for R3 and a high value for R+ and/or RF.

The solution follows

For the particular case of a unity-gain follower, RF is normally 0Ω as shown in Fig 3. According to the foregoing general analysis, if the value of RS is low, ac noise gain is (R4/R5)+1, so you can increase ac noise gain—and therefore stability—by adding a large R4 and a small R5. (A large and constant RS can make R4 unnecessary; ac noise gain is then (RS/R5)+1.)

Improved unity-gain follower delivers fast, stable response figure 3
With LM110/LM310s, for example, 10k is an appropriate value for R4. Using R5 = 3.3k and C5 = 200 pF, the LM110 stably drives capacitive loads up to 600 pF.

Technique speeds followers
You can also wire the resistor/capacitor combination across an op amp's inputs to increase the follower's slew rate. For example, an LF357 op amp's decompensation with a small internal capacitor normally requires gains higher than five to maintain stability (Fig 4). But the LF357 fits unity-gain-follower applications as easily as the LF356 (which is identical to the 357 except for the 356's internal compensation) and achieves better results. When source resistance is less than 1k, both the LF357 and 356 provide fast, stable responses, but the 357 has a 50V/μsec slew rate (typical) compared with 12V/μsec for a 356.

Improved unity-gain follower delivers fast, stable response figure 4

The LM349 decompensated quad op amp furnishes a bipolar input stage with a finite bias current (200 nA max). For best results in this application, add the resistor that controls the noise gain equally to the inverting and noninverting inputs as shown in Fig 5. With this circuit, the LM349 can slew at 2V/μsec typ and handles audio signals much faster—and without distortion—than the compensated LM348 (which, at 0.5V/μsec, slews only as fast as the general-purpose LM741). You can use the same approach for an LM101 by employing a 5-pF damping capacitor.

Improved unity-gain follower delivers fast, stable response figure 5

Watch for problems

While inserting a resistor/capacitor combination across the inputs gives faster slewing, the circuit's bandwidth could degrade if source impedance (RS) increases. In addition to guarding against bandwidth problems, make sure ac noise doesn't reach an objectionable level when you raise ac noise gain. Although most modern op amps exhibit low noise, raising that noise gain to 10 can significantly increase output noise.

If the series capacitor across the op amp's inputs is larger than necessary for stability and high slew-rate purposes, noise increases unnecessarily. In general, choose the minimum capacitance for the circuit in Fig 3 according to the following formula (where fV = op amp's unity-gain bandwidth):

C5min = 4[1+(R4/RS)]/2πR5fV=(R4+R5)/(π/2)fV(R5)2. To allow for tolerance variations, make C5's circuit value two or three times C5min.


Bootstrap Buffer Based Preamp

I have been playing with a number of variations of the circuit as a free standing preamp, headphone amp, 321/729 replacements and as a potential active crossover, any and all of which may happen if there is demand. There has been strong interest in all of the threads and my bootstrap buffer preamp is up and running nicely - and for the moment it's staying. The same board should also be very handy as a headphone amp - & I'll be trying that next week.
They are different animals in so many different ways and come from completely different design philosophies. . and these are only my own observations and conclusions - so Jiim & PD feel free to chip in...please!

The starfish is definitely and very firmly in the Naim mold...and I love it for many things...lots of bass, drive and pace it really rocks..., but I can't help feeling that it's adding it's own signature to the sound. It's obviously based on maximizing the potential of the classic Naim 321 and 729 circuits and that means sorting a complex grounding scheme and multiple local regulators for power supply sensitive circuits. Accordingly it has a large and somewhat expensive BOM and is a fairly complex build.

The Bootstrap on the other hand is so much simpler and follows a minimalist design approach with as few components as possible in the signal path - it also uses symmetrical power rails and minimizes ground points in the signal path. So the BOM is smaller, the build easier and much more economical - I'll post a BOM in the next day or so. The result in the early listening is a cleaner sound with incredibly low noise and distortion, and I believe a much more truthful and honest presentation of the source material with uncanny staging and detail retrieval.

It's my expectation that I'll end up using both, the Starfish in a larger system (active Briks..) and mainly for rock & large scale orchestral music, the Boostrap in a smaller system where I'm looking for more space and precision, blues, jazz and acoustic programme material.

I guess that one of my conclusions is that just as no one system will please everyone, no one system is best for reproducing all types of material - at least not for my ears.


 Bootstrap Buffer Based Preamp

Bootstrap Buffer Based Preamp Single Rail Supply

TURBO: A series of bipolar transistor amplifiers

25 Watt to 100 Watt, the range of TURBO is broad enough to satisfy everyone's needs.
These amplifiers are still valid and will satisfy lovers of the "bipolar" and those wishing to embark on the realization of a way "serious" quality.

In November 1980, Dominique JACOVOPOULOS published in Radio plans a series of bipolar amplifiers TURBO .

Here are excerpts:

* Amplifier TURBO 50

The amplifier TURBO 50, 50 Watt/8Ω

The article is available:

* Amplifier TURBO 75
 The amplifier TURBO 75, 75 Watt/8Ω

The article is available:

* Power supply

This power supply is adjustable from 30 to 40V/3A and allows to connect the two channels of a TURBO 50 and TURBO 75.

The article is available:

* Amplifier TURBO 100

Turbo 100 output power 100 Watt/8Ω

In January 1981, published in Radio Plans # 398 the 100 Turbo.

The article is available:

* Amplifier TURBO 25

 TURBO amplifier 25 Watt/8Ω

The article is available:

TURBO 25 regulated power supply (polarity shown here)

Chemical charge a capacitor to smooth voltage at 100 Hz has too much work to undergo more random shocks for a long time. Its longevity depends greatly on operating conditions, performance related to them.
TURBO in the diet, the filter head C 1 assumes the thankless role.

But the sound is extracted from C6 is charged at constant voltage by the voltage stabilizer interposed between these two chemicals. Here we have a system with rapid transfer of energy under the control of the valve that is the electronic ballast transistor. This ensures the best performance in chemical output that does not suffer as current variations.

Extract from Article TURBO 25

Plans Radio 403, 06/1981

50 watts transistor amplifier

The amplifier and speakers that can handle medium-power is designed to provide a strictly amateur. Accidental overloads can damage the speakers, it is not appropriate for small systems.

What amp settings do not contain an element of the first connection wiring must be careful to work with.
Characteristics of the transistor, the fan or heat sink is cooled enough to find out if you need to focus!

Tech. parameters:
Power: + - 28V
Power: 50W / 4 ohms
Input sensitivity: 250mW of
Input resistance: 50 kOhm
Frequency range: 30Hz to - 30kHz

Optimal mobile recording portable player to another amplifier Multi Media.

Here, the schematics this power amplifier

List of components:
R1, R2, R9 - 56K
R3 - 3K3
R4, R6 - 100R
R5 - 220R
R7, R8 - 120R
R10 - 1K
R11 - 1R
C1 - in 1μF / 35V
C2 - 33P - Ceramics
C3 - the 100μF/35V
C4 - 100 N (220N) - Ceramic
C5, C6 - 4.7 UF / 35V
D1, D2 - 1N4007
T1, T2, T9 - BC546
Q3 - BC640
T4 - BD139
T5, T7 - BD711
T6 - BD140
T8 - BC639

Following the DC voltage amplifier and limiter speaker protection is needed.


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