APPLICATION BULLETIN ® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 CLAMPING AMPLIFIER TRACKS POWER SUPPLIES By Jerald Graeme (602) 746-7412 clamps output eO by diverting feedback current away from R2. This occurs when eO reaches a level sufficient to turn on either Q1 or Q2. Then, a transistor collector current absorbs any additional signal current supplied through R1. No further signal current reaches R2 and this limits the level of eO. Clamping amplifiers limit signal magnitude to protect following circuitry from input overload. The clamping amplifier shown here produces limit levels that track the power supply levels selected in any given test configuration. Conventional clamping amplifiers set limit levels referenced to ground, rather than the supply levels, and restrict signal swing to the minimum available under minimum supply conditions. However, the clamping amplifier shown automatically adapts the clamp levels to supply changes, maximizing the allowable signal swing. Zener diodes DZ1 and DZ2 primarily determine the power supply and clamp level relationships. These zeners establish voltage levels with fixed separations from the supply voltages. Diode DZ1 biases the base of Q1 at V– +VZ1, and DZ2 sets the base of Q2 at V+ –VZ2. These base biases set the clamp transistors for turn on at specific clamp levels. A negative going eO turns on transistor Q1 when eO reaches a level of VL– = V– +VZ1 – VBE1 – VD1. Then, Q1 conducts current through its collector, diverting any additional feedback current away from R2. Any further increase in ei magnitude simply supplies more current to Q1 rather than to R2. This holds the circuit output voltage at the turn-on level VL–. Similarly, a positive going eO turns on Q2 when eO reaches the positive limit of VL+ = V+ – VZ2 + VBE2 + VD2. Traditional clamping amplifiers produce output voltage limits referenced to common. However, the input overload levels of most circuits depend upon voltage levels referenced to the power supplies. There, the internal bias voltage requirements of a circuit define minimum voltage separations between the signal and the power supply levels. This references the overload levels to the supplies, rather than to ground. Ground-referenced limits adequately accommodate these bias requirements where the power supply levels remain relatively fixed. Then, ground-referenced limits simply subtract fixed amounts from the worst-case, low supply levels. With the components shown, VZ = 6.2V and VBE and VD = 0.6V producing 10V output limiting for 15V supplies. Tolerance variations in the component voltages introduce a 400mV worst-case error to the clamp voltages. No significant clamp-level error results from the precision OPA77 shown. However, power supply levels vary greatly in test systems where control signals set the supply levels. Then, the worstcase setting of ground-referenced limits often sacrifices operation in otherwise safe signal ranges. The clamping amplifier shown adapts to supply variations by referencing the clamp trip points to the supplies rather than to ground. Higher supply voltages then produce higher clamp levels. This avoids lost signal range by adapting the clamping limits to the varying supply versus input capabilities of the following circuit. However, the clamp circuit adds gain in the feedback loop, compromising feedback stability. When one of the clamp transistors conducts, it acts as a common-base transistor in the feedback loop. This adds a gain of (R1 || R2)/RE to the open-loop gain of the amplifier. Here, RE represents the impedance of the transistor’s emitter circuit and this impedance is quite low. The emitter circuit impedance includes the dynamic emitter impedance of the transistor and the forward impedance of the diode. Feedback analysis1 shows that this added gain shifts the net open-loop gain upward, exposing a region of two-pole response roll off. Lower closed-loop gains encounter this stability-compromising region. For those cases, the capacitor shown rolls off the load impedance of the common-base transistors, removing the added gain at high frequencies. Basically, the circuit shown consists of an inverting amplifier formed with the op amp, R1 and R2. The remaining components produce the clamping action, provide breakdown protection, and phase compensate the circuit. The zener diodes, transistors and R3 establish the basic clamp reference voltages. Diodes D1 and D2 protect the transistors from emitter-base breakdown and the capacitor compensates the feedback stability complicated by the clamp. The circuit © 1993 Burr-Brown Corporation AB-054 Printed in U.S.A. April, 1993 15V V+ DZ2 D1, D2:1N4154 DZ1, DZ2:1N4627 2N2907 C 30pF R1 10kΩ Q2 D2 R2 10kΩ R3 68kΩ ei eO eO OPA77 VL+ D1 ei 2N2222 VL– Q1 DZ1 VL+ = V+ – VZ2 + VBE2 + VD2 = 10V VL– = V– + VZ1 – VBE1 – VD1 = –10V –15V V– FIGURE 1. The limit levels of a clamping amplifier track the power supply levels when zener diodes establish fixed voltage differences between the two levels. REFERENCE 1. Graeme, J. G., Feedback Plots Define Op Amp AC Performance, Burr-Brown Application Bulletin AB-028, 1991. The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. 2