diff --git a/makefile.in b/makefile.in
index cd8b6108a..f5526b8e4 100644
--- a/makefile.in
+++ b/makefile.in
@@ -523,8 +523,8 @@ filter_autotests :=						\
 	src/filter/tests/iirdecim_autotest.c			\
 	src/filter/tests/iirdes_autotest.c			\
 	src/filter/tests/iirdes_support_autotest.c		\
+	src/filter/tests/iirfilt_autotest.c			\
 	src/filter/tests/iirfilt_xxxf_autotest.c		\
-	src/filter/tests/iirfilt_copy_autotest.c		\
 	src/filter/tests/iirfiltsos_autotest.c			\
 	src/filter/tests/iirhilb_autotest.c			\
 	src/filter/tests/iirinterp_autotest.c			\
diff --git a/src/filter/src/iirfilt.proto.c b/src/filter/src/iirfilt.proto.c
index a74e3056c..c89cbfa82 100644
--- a/src/filter/src/iirfilt.proto.c
+++ b/src/filter/src/iirfilt.proto.c
@@ -350,8 +350,8 @@ IIRFILT() IIRFILT(_create_differentiator)()
         0.1958670f * cexpf(_Complex_I * M_PI / 180.0f *  -40.51510f),
         0.1958670f * cexpf(_Complex_I * M_PI / 180.0f *   40.51510f),
         0.1886088f,};
-    // gain, digital, differentiator
-    float complex kdd = 2.09049284907492e-05f;
+    // gain, digital, differentiator (slight adjustment added for proper gain)
+    float complex kdd = 2.09049284907492e-05f / 1.033477783203125000f;
 
     // second-order sections
     // allocate 12 values for 4 second-order sections each with
diff --git a/src/filter/tests/iirfilt_copy_autotest.c b/src/filter/tests/iirfilt_autotest.c
similarity index 86%
rename from src/filter/tests/iirfilt_copy_autotest.c
rename to src/filter/tests/iirfilt_autotest.c
index a4e3d85d3..20719fdab 100644
--- a/src/filter/tests/iirfilt_copy_autotest.c
+++ b/src/filter/tests/iirfilt_autotest.c
@@ -23,7 +23,59 @@
 #include "autotest/autotest.h"
 #include "liquid.internal.h"
 
-//
+void autotest_iirfilt_integrator()
+{
+    // options
+    unsigned int num_ones    = 10;
+    unsigned int num_samples = 40;
+
+    // allocate memory for data arrays
+    float buf_0[num_samples]; // filter input
+    float buf_1[num_samples]; // filter output
+
+    // generate input signal
+    unsigned int i;
+    for (i=0; i<num_samples; i++)
+        buf_0[i] = i < num_ones ? 1 : 0;
+
+    // create integrator and run on sample data
+    iirfilt_rrrf q = iirfilt_rrrf_create_integrator();
+    iirfilt_rrrf_execute_block(q, buf_0, num_samples, buf_1);
+    iirfilt_rrrf_destroy(q);
+
+    //for (i=0; i<num_samples; i++)
+    //    printf("[%3u] %20.17f\n", i, buf_1[i]);
+
+    // check that last value matches expected
+    CONTEND_DELTA(buf_1[num_samples-1], num_ones, 0.01f);
+}
+
+void autotest_iirfilt_differentiator()
+{
+    // options
+    unsigned int num_samples = 400;
+
+    // allocate memory for data arrays
+    float buf_0[num_samples]; // filter input
+    float buf_1[num_samples]; // filter output
+
+    // generate input signal
+    unsigned int i;
+    for (i=0; i<num_samples; i++)
+        buf_0[i] = i;
+
+    // create differentiator and run on sample data
+    iirfilt_rrrf q = iirfilt_rrrf_create_differentiator();
+    iirfilt_rrrf_execute_block(q, buf_0, num_samples, buf_1);
+    iirfilt_rrrf_destroy(q);
+
+    //for (i=0; i<num_samples; i++)
+    //    printf("[%3u] %20.17f\n", i, buf_1[i]);
+
+    // check that derivative is equal to 1
+    CONTEND_DELTA(buf_1[num_samples-1], 1.0f, 0.01f);
+}
+
 void autotest_iirfilt_dcblock()
 {
     // options
@@ -68,30 +120,6 @@ void autotest_iirfilt_dcblock()
     iirfilt_crcf_destroy(filter);
 }
 
-void autotest_iirfilt_integrator()
-{
-    // options
-    unsigned int num_ones    = 10;
-    unsigned int num_samples = 40;
-
-    // allocate memory for data arrays
-    float buf_0[num_samples]; // filter input
-    float buf_1[num_samples]; // filter output
-
-    // generate input signal
-    unsigned int i;
-    for (i=0; i<num_samples; i++)
-        buf_0[i] = i < num_ones ? 1 : 0;
-
-    // create integrator and run on sample data
-    iirfilt_rrrf q = iirfilt_rrrf_create_integrator();
-    iirfilt_rrrf_execute_block(q, buf_0, num_samples, buf_1);
-    iirfilt_rrrf_destroy(q);
-
-    // check that last value matches expected
-    CONTEND_DELTA(buf_1[num_samples-1], num_ones, 0.1f);
-}
-
 void testbench_iirfilt_copy(liquid_iirdes_format _format)
 {
     // create base object